However, he reformed in Alien Force as apart of Ben's team, and then became one of the main hero characters. However, following the events in The Forge of Creation, he becomes an insane villain again until his sanity was restored in Absolute Power Part 2.
In the Original Series, Kevin was a thin and scrawny boy with shoulder-length black hair, pale skin and dark brown eyes with black-discolored marks around them. He wore a torn black t-shirt, black studded arm bands, brown cargo shorts, gray knee-length boots.
In Alien Force, Kevin had grown into a tall and muscular young man. His eyes now lack the black marks (as his body fully recovered from and stopped absorbing energy). His normal attire is a tight black t-shirt over a gray long sleeve shirt, blue jeans, and black combat boots. His hair, though still long, is styled into a choppy mullet. Occasionally, he is seen wearing a light blue, oil stained, mechanic's jacket over a white muscle shirt, blue jeans, and his black combat boots.
When Kevin was 11, he was a criminally insane sociopath, willing to hurt/kill anyone for his own personal gain and/or pleasure. This behavior and mentality stems not only from being seen and treated as a freak by his peers and family, but also because of the side effect of Kevin absorbing energy. His selfishness and sociopath-nature is made apparent when he was once willing to derail a money train to steal the money and kill countless passengers on another train. Kevin had an extremely negative outlook on life because of the countless hardships and struggles he had, feeling insecure about himself as well as having incredibly low self-esteem. He had a deep-rooted hatred for other people and society in general, even the innocent as he once said "Nobody's innocent! They just haven't had the chance to make fun of me yet!".
While changing for the better in Alien Force and Ultimate Alien, Kevin still sometimes shows a delinquent side, as seen in Kevin's Big Score, where he attempts to sell the Rust Bucket to Vulkanus for alien technology, albeit technology he intended to give to Ben and conspires with Argit to sell weapons to both sides of a warring race in Simple. Despite this side of Kevin, Ben and Gwen still show great trust in him.
Kevin was first encountered by Ben in an arcade, where, after befriending him, he was assaulted by bullies and saved by XLR8. The two attempt to steal a video game from a warehouse and got caught by the police, forcing Ben to reveal the Omnitrix to him in order to escape. The two subsequently form a partnership to use their respective powers for personal gain; however, when Kevin rigs two trains (one carrying money and one carrying people) to collide, uncaring that the crash would most likely result in the deaths of several hundred people, Ben suffers a crisis of conscience and breaks off their partnership. Ben became Heatblast, wanting Four Arms, to stop Kevin, but to even the odds, Kevin grabbed onto Heatblast and absorbed his DNA, changing him into an incomplete version of Heatblast, called Fireblast. When Heatblast foils Fireblast's plans and escapes, Fireblast leaves to use his powers to get revenge on the bullies that tormented him. But Fireblast's powers disappear and he reverts back to Kevin. When Four Arms showed up to stop Kevin, Kevin absorbed Four Arms' DNA, changing him into an incomplete version of Four Arms. As Four Arms and Kevin, Four Arms gains the victory and offers Kevin a new home and a chance to be a good guy. Kevin, but Ben realizes he lied and Kevin tries to take the Omnitrix off of Ben, but feedback from it changes Kevin back to normal and appears to have removed his Osmosian powers. He escapes and at the end of the episode reveals he still has Heatblast's powers.
Kevin serves as the main antagonist of the second season. His ability to project heat at the end of Kevin 11 is the result of him absorbing the Omnitrix's feedback: the amount of energy it emitted allowed him to fully absorb its powers, giving him access to every unlocked alien form. However, because Kevin's ability to regulate this power is poorer than that of the Omnitrix itself, the energy suppresses his real form and forces him to remain as an alien most of the time. As revealed in Framed, Kevin blamed Ben for this, so he used the powers of the Omnitrix to start a inter-state crime spree in America, both for his own benefit and to damage Ben's (or rather, his alien forms') reputation. Eventually, Kevin's ability to regulate the power fails altogether, causing him to mutate into what Vilgax describes as a "misshapen, chaotic amalgam of creatures from the Omnitrix.", which Kevin calls Kevin 11. Because of this, Kevin 11 fights Four Arms and as he was about to kill him, Lt. Steel from the Special Alien Capture Team (SACT) shoots him into the water and he is defeated, but at the end of the episode, he is revealed to be alive.
Kevin 11 gloats that he has all of Ben's powers plus his own, making him Kevin 11. The known powers he could use include Four Arms's strength, Ripjaws's underwater breathing and steel-bending jaws, Upgrade's stretching, Stinkfly's flight and slime spit, Diamondhead's crystal projections, shape-shifting arm. and immense durability, Heatblast's pyrokinesis, Wildmutt's sense of smell, and XLR8's speed and his Osmosian powers. Despite this, Diamondhead observes that Kevin 11's powers weren't as strong as their pure forms, exactly 1/10th according to him.
In Grudge Match, Kevin 11 attacked Ben to get revenge, blaming his condition on him. As the two battled, they were abducted and forced to fight in Slix Vigma's gladiator games. During their battles, Ben helped Kevin 11 to realize the advantages of his mutated form: because Kevin 11 was an amalgam of aliens, he could mix and match their powers to make up for their reduced potency (Diamondhead fist + Four Arms strength + XLR8 speed = "one mean punch"). Although forced to work together, Kevin 11's only goal was killing Ben. After Ben and Kevin 11 succeeded in escaping, Kevin 11 attacked Ben and trapped him against the wall with Stinkfly's goo before moving in with Diamondhead's spear hand, gloating that no matter what alien Ben transformed into, he knew every one of them. Little did he know that Ben unlocked Cannonbolt, whom he transformed into just as Kevin 11 was about to murder him before getting into an escape pod. Kevin 11 nearly barged into the pod when he was stopped by Technorg, whom Ben had spared and he declared a life debt to. After Technorg helped Cannonbolt escape, he turned his attention to Kevin (who had called him a lap dog just as he was about to kill Ben). The two were transported to a different galaxy at the end of the episode while locked in combat. Technorg defeated Kevin 11 and left the ship to return to his home world while Kevin 11 took the ship as his own (according to the advanced edition of the episode).
Kevin 11 returned in Back With a Vengeance, where he teamed up with Vilgax to defeat Ben, and eventually managed to remove the Omnitrix with Vilgax's help. However, he betrayed Vilgax in an attempt to leave both Ben and Vilgax in the Null Void and use the Omnitrix to take over the world, only for Ben to escape and leave him and Vilgax trapped instead. Kevin spent the rest of the series in the Null Void.
In his first Alien Force appearance, Kevin attempted to organize a deal between the Forever Knights and the DNAliens in order to sell them level 5 weapons. However, the deal was interrupted by Magister Labrid, Ben and Gwen. Trying to get revenge, Kevin fought Ben, but was easily defeated while the other villains escaped without paying him. Labrid then forced Kevin to help them in finding back the weapon, offering him to avoid problems with the Plumbers then. Though Kevin mainly accepted in order to get his pay, Magister Labrid later sacrificed himself in order to save him from a defective weapon's explosion. This, plus Kevin's interest for Gwen, his promise to Magister Labrid, and his desire to start a better life and become a Plumber like his father, led him to agree to teaming up with Ben and Gwen against the High Breed invasion. It "mostly" served as the reason that he joined the Tennysons in the first place (his promise to Magister Labrid and Gwen could be the other reasons).
Though he initially still felt a little rancorous toward Ben, Kevin slowly reconciled with him, and he proved to be a valuable ally as well as a good friend. Despite his reformation, Kevin still tends to engage in unethical activities: he is not above stealing gold and jewelry from the Forever Knights and Plumbers equipment from Manny and Helen when presented with the opportunity. In Darkstar Rising, Kevin reveals that his father was a Plumber. When young, his mother told Kevin stories about what his father did. This is a big gap in the original series because in Ben 10, he said that his parents both left him because of his Osmosian powers; this was explained later on in the series as an exaggeration on Kevin's part: Kevin's mother remarried, and in retaliation Kevin destroyed their home which lead to his stepfather causing a huge strain between Kevin and his parents, ultimately making Kevin think they hated him and ran away. At the end of the episode, Kevin, along with Gwen and Ben became unofficial Plumbers, after which Kevin leaves to tell his mother about his membership, meaning he has/wants to reconcile and rejoin with his family. When explaining that he wanted to be a Plumber for the sake of being like his father, it "mostly" served as the reason that he joined the Tennysons in the first place (his promise to Magister Labrid and Gwen being the other reasons).
In Vengeance Of Vilgax, Kevin was subjected to another feedback pulse from the Omnitrix after Ben tampered with it. Because of this, he was unable to revert back to his normal form, instead becoming a living colossus composed of different matters. Throughout the season, he was shown to hate what he had become, to the extent of breaking things off with Gwen (much to her dismay), wearing an I.D. Mask in public to appear human, and considering himself a monster. He also discovered that his father was murdered by Ragnarok, and became bent on vengeance, so much so that he eventually left Ragnarok to his death when he had the chance to save him.
In The Final Battle, Vilgax attempted to separate Ben from his friends, Kevin was kidnapped by Albedo in an attempt to split them up. He was also asked by Ben to back him up when taking on Vilgax, which he did. After the Omnitrix was destroyed, he reverted back to normal, revealing that it was the Omnitrix that kept him that way. This is also when Kevin and Gwen share their first onscreen kiss on the lips.
Ben 10: Ultimate AlienEdit
Kevin and Ben no longer share their famous rivalry and instead have become true friends. Kevin acts as an older brother towards Ben. They care genuinely for each other. Kevin is much more open about his feelings, showing gratitude and tries to express it to both Ben and Gwen (mostly Gwen). Gwen and Kevin continue their romantic relationship. He also seemed to have a better connection towards Max.
In Too Hot To Handle, Kevin is shown to be able to repair objects in the current matter he has absorbed by releasing a liquid version of that substance and then hardening it again. He can also manipulate whatever matter he is touching by absorbing it and then morphing it into any shape or form he wants. He uses both abilities when he fixed the broken seal of P'andor's suit, morphed it around his body to trap him, and fused the arms together as makeshift handcuffs.
In Fused, Kevin admits he was selfish when he first met Ben. He also risked his life to save Ben from being taken over by Ra'ad. He also learns that there is another Osmosian named Aggregor. While never explained, Kevin shows sympathy to him because they are, somewhat, the same race and because Kevin, like Aggregor, was once an insane and violent sociopath.
In Ultimate Aggregor, Kevin shows his knack for detective work, and later warned by Paradox to remember who his friends are and that things will get worse before they get better. Kevin explains to Aggregor that the reason that he was a villain in the original series is that when Osmosians absorb energy, they become insane, despite being true, Aggregor disagrees.
In Map of Infinity, Kevin continues to focus on stopping Ultimate Aggregor, Kevin is easily defeated by Ultimate Aggregor, Kevin also seemed to know about the Map of Infinity saying it was just a legend, the group go in search of the first part, Kevin wears a Plumbers' Suit, unlike any other Plumbers' Suits, that Kevin had a little white glove and a circle over the glove (which gives Ben access to the Ultimatrix), possibly this glove is for Kevin to use his powers, he proved to be very strong when a giant hammer crushes the group on a wall, he survived and managed to save the group and remove the hammer from the wall.
In Perplexahedron, when Ben and Kevin get separated from Gwen, Ben says that Gwen can take care of herself and Kevin replies, "That's why I like her." Kevin then talks about his feelings to Ben and how he owes it to Ben and Gwen changing his life. After finding Gwen, Kevin tries to say that he likes her. Then Swampfire tells her what Kevin's trying to say, and Gwen says, "I know that already." and they kiss.
In The Forge of Creation, as a last resort to stop Ultimate Aggregor from succeeding, Kevin absorbs the powers and abilities from the Ultimatrix's unlocked aliens, becoming Ultimate Kevin, powerful enough to defeat Ultimate Aggregor. Unfortunately, this also causes him to become insane once again. His old desire for power also returns as he absorbs Ultimate Aggregor's powers as well. However, Kevin's good heart appears to not be taken over completely, as he accepted 10-year-old Ben's harsh criticism before sparing Aggregor's life, though he declares he will not be so merciful tomorrow.
In Nor Iron Bars a Cage, it is revealed that back during Kevin 11's time in the Null Void, a prisoner named Kwarrel approached him and offered to help him control his anger, revert to normal and find a purpose, after that he learned how to absorb matter. Kwarrel then showed Kevin a tunnel he had been digging out of the Null Void. However, they were ambushed by Morgg, who would one day become the warden. Kwarrel held off Morgg for Kevin to escape, only to lose his life in the process. Out to avenge his death, even though it went against all that Kwarrel had taught him, Ultimate Kevin purposely returned to the Null Void to kill Morgg. He would have succeeded if Gwen didn't save Morgg. Believing that Kevin is truly gone, Ben, much to Gwen's horror, suggests that Ultimate Kevin must be put down.
In The Enemy of My Enemy, Ultimate Kevin went after Argit for all the times he double-crossed him. He first attacked him and sent him towards Ben and Gwen. He later attacked Rust Bucket 3 in order to get Argit, but was forced to retreat due to his car being thrown off the plane. He then attacked the Plumber's Academy and easily took care of the rookie Plumbers. He then attacked and thought he killed Argit (due to Argit closing down his pulse). During his attempt to kill Argit, he nearly killed Gwen, thinking she betrayed him. However, he restrains himself and spares her life, saying: "The only reason you're alive is because of what you used to mean to me." before departing, making is clear that a part of Ultimate Kevin still cares about her and Gwen remaining adamant that they can still bring Ultimate Kevin back to sanity without killing him.
In Absolute Power, in hopes of trying to save Kevin, Gwen inquires the aid of Darkstar. However, Ben is very furious with the idea. This is Gwen's last hope of trying to bring Kevin back. Gwen tries to talk to Ultimate Kevin, who tells her he's been avoiding her because he can't control himself, implying that he doesn't want to hurt her; but he gains the desire to absorb Gwen's powers.
After Ultimate Echo Echo defeated Ultimate Kevin, Cooper finished the machine that would siphon all of the energies that Kevin had absorbed and subsequently cure him with it. Darkstar then betrays the team by using the piece of the Dominus Librium to transfer all said energies into himself, which backfires when Ben; anticipating such a move, activates a switch to return all of the energy back to where they belong, Kevin then knocks him out with a punch. Kevin thanks Cooper. He is then given a passionate kiss on the lips by Gwen (much to the disappointment of Cooper) and gets into an argument with Ben about their previous fight, to which he is relieved that everything turned out alright, after which Gwen asks for their help in explaining to her parents about the damage done to their home, to which Kevin and Ben ditch by voting to go to Mr. Smoothies instead, with Kevin buying. In Eye of the Beholder, Kevin was eating a Fling Cake that was considered the most delicious desert in the galaxy. After Ben caught up with him and the girls on Monok , Kevin wasn't impressed with Ben blowing Julie off like a jerk and explained to him that if he never acted like a jerk she wouldn't have dumped him. After Julie returned The Sentinel's missing eye, he rephrases Ben's question with the word "in" instead of "on" and has Ben talk to Julie and the two became a couple again.
Originally, in Ben 10, Kevin's powers were believed to be because of an unexplained mutation but later explained to be of alien origin. This was proven correct in Vendetta, were it is revealed that he is an Osmosian descendant. Kevin has the unique ability to absorb the energy and matter of anything (excluding certain materials as seen in Greetings From Techadon) though his body at will and utilize it in various ways. This includes, but is not limited to, electricity, DNA, memories of other life forms, powers of other life forms, mana, fire, water, plasma, chemical energy, kinetic energy and matter.
After absorbing energy, Kevin can channel energy through his hands to manipulate technology, discharge it as an energy blast or use it to speed up cellular regeneration to increase his strength or recover from injuries at an accelerated rate. However, the energy will dissipate over time or through excessive use, so he is required to recharge.
If Kevin was to absorb energy, the energy will cause Kevin to become mentally unstable. Storing energy for long periods of time will worsen his instability. When Kevin has recently absorbed energy, he develops black marks around his eyes. This is a side effect of absorbing energy.
Similar to his ability to absorb energy, by absorbing the DNA, metabolism, and life force of other life forms, Kevin can use it as nourishment to sustain his health and strength. Absorbing all of someone's energy will kill them. At higher levels, Kevin can use the energy to acquire the creature's powers and undergoes mutation to accommodate the absorbed abilities.
Despite complete access to any of these absorbed abilities at anytime, they aren't as strong as their pure forms, as noticed by Ben who speculated that they only are about 1/10 their full power, as seen when Diamondhead was able to catch and crush Kevin 11's crystals with his bare hands while the others shattered upon hitting him. This was explained in Ultimate Alien by Aggregor. He also stated that when Osmosians absorb the life force of other beings, the new powers can only used temporarily unless they were to drain all of that life form's energy, including their life force.
Kevin can absorb the properties of solid materials, transmuting himself into a living form of whatever material he has absorbed and subsequently use it to proportionately increase his physical strength and durability. It is revealed in All That Glitters, that the transmutation serves as a second skin that can chip away. In Plumbers' Helpers, Kevin was able to absorb multiple materials at once. He requires a decent amount of density or mass in the material to completely encase himself (a marble can only produce enough to cover his hand while a Taedenite crystal, the size of his hand, is enough to cover his entire body). This ability has once been used productively when Kevin was forced to synthesize a coating of Taedenite, thus incapacitating him; Paradox mentioned in Vilgax Attacks that with the amount Vulkanus chipped off of Kevin is now the largest stash in the galaxy.
In Season 3 of Alien Force, following Kevin's second mutation caused by the feedback from tampering with the Omnitrix, he loses the ability to revert to his human form but gradually begins learning how to use his absorbed matter in various new manners. He becomes able to use high levels of strength and durability as well as limited regenerative and shape-shifting abilities, seen when he shifts his limbs into a variety of different tools and weapons and restoring his severed hand. Despite returning to normal in "The Final Battle: Part 2" after the Omnitrix self-destructed, Kevin shows in Ultimate Alien to retain his heightened matter manipulation powers due to him now having knowledge of being able to doing so.
In Too Hot To Handle, Kevin's abilities have shown to have developed more, allowing him to repair objects of the current material he has absorbed either by turning it into a liquid or releasing a liquid version of whatever material he has absorbed. He can partially merge with the actual matter and manipulate the substance's form. Both abilities were used when he repaired the rock ceiling of a mine, fused the seal of P'andor's suit, morphed it around P'andor, and fused the arms together as makeshift handcuffs.
In The Creature From Beyond, he used the metal he absorbed to create a helmet for Gwen to prevent the Lucubra from controlling her. Kevin can easily pick locks, using his powers to absorb the material of the lock, then form his finger into a key. Kevin can also absorb liquids, and when he does he gains the properties of that liquid (according to Dwayne, if Kevin absorbs mud, objects will pass through him without causing physical damage).
As seen in The Purge, if a part of his body was severed while he had absorbed some matter he could replace his hand.
Even without his absorption powers, as an Osmosian, Kevin is shown to be stronger than an average human. While not to the same extent as aliens like Four Arms, Kevin is capable of lifting throwing the combined weight of several people with little strain. He is also capable of resisting impacts and blunt force trauma that would severely injure an ordinary person with moderate discomfort.
Kevin has some noticeable skill in martial arts which he gained during his years as a runaway, a prisoner in Incarcecon, an alien tech dealer, and a member of the Plumbers. With this combat skills, he has proven on several occasions to be a highly formidable fighter, having held his own against and defeated opponents such as the Megacruiser robots, Technorg, and even Vilgax in a one-on-one fight, something that Ben couldn't do.
In Deep, Ben and Kevin argue that they both cannot use their powers while being in their Plumber suits. So Kevin made some alterations to the suits, equipping them with propelling units and shock pulses that allow them to attack with electric shocks. Kevin also has great analytical and deductive skills, having been able to determine specifically what happened in a battle between Aggregor and the Plumbers several hours after the battle, greatly impressing his friends and even gaining a compliment from Max.
Kevin is a very good driver (both on land or air), being able to race and chase in his car and the Rust Bucket 3. He is also a great at mechanics. He is shown to repair his car for the numerous damage it took during the course of the series. He also modified his car, setting alien tech for beneficial gears. On top of that he built the DX Mark 10, a car that he gave to Ben as his sixteenth birthday present.
- A running gag in the series is how Kevin's car gets damaged or destroyed in one way or another during battle. It would seem that the Rust Bucket 3 has also become a part of this running gag.
- In the episode The Widening Gyre it is indirectly stated that Kevin watched "Captain Planet" and "X-Files" when he was younger.