He has appeared in every current episode, and is the only character to do so.
He is first introduced in the pilot episode And Then There Were 10, where he finds the Omnitrix and then, throughout the series, learns to use the immensely strong and powerful alien device, including the great responsibility of wielding it wisely to become a true hero.
Ben 10 (Original Series)Edit
Ben was originally a ten-year-old boy, who grew up in Bellwood, and was unpopular and bullied in his school.
However, his life changed the day he went on summer vacation with his grandfather, Max and his cousin, Gwen. A short time after they left Bellwood, Ben went on a walk through the woods in which they were camping, and witnessed the crash of a strange capsule from space. This capsule happened to contain the Omnitrix, an immensely strong and powerful watch-like device, that firmly affixed itself onto Ben's left wrist.
Despite being very immature and occasionally using the for minor selfish purposes, Ben proved to be a valuable wearer for the Omnitrix, defeating Vilgax several times and saving people on multiple occasions.
Ben 10: Alien ForceEditThe sequel Ben 10: Alien force continues Ben's adventures five years after the original series. At some point between the two shows, Ben, now fifteen years old, had succeeded in removing the Omnitrix through an unknown but difficult process, and chose to retire from his superhero career in order to have a normal teenage life, though he kept the unactivated Omnitrix as a souvenir. He then grew up into a slightly more mature teenager, loosing his unpopular status at school and becoming a skilled soccer player.
Soon Max mysteriously disappeared while investigating on alien newcomers on Earth known as Highbreeds, leaving only a enigmatic holographic message for Ben. Despite Gwen's doubts, Ben decided to use the Omnitrix again in order to find Max. When he put it back, the watch recalibrated, taking a new appearance and providing him with a brand new set of ten aliens. These ten new aliens possess special abilities and powers that are much stronger and much more powerful than his original ten.
Ben 10: Ultimate AlienEditIn Ben 10: Ultimate Alien, sixteen-year-old Ben's identity is revealed to the world and he's now a megastar superhero loved all over by kids, but distrusted by many adults. This series features Ben's new car, the DX Mark 10 a heavily modified Mazda RX-8, which marked it's first appearance in Ben 10: Alien Swarm, the car being gifted reluctantly by Kevin. Also he will be seen with Julie, now sharing a stronger bond with him and Julie again helping their team from time to time.
Also, Ben and Kevin are now true friends and not rivals. Ben is now armed by an enhanced,upgraded version of the Omnitrix, the Ultimatrix, which evolves his aliens.
Ben 10: OmniverseEdit
In Ben 10: Omniverse, Ben is still 16 years old, and often fights side-by-side with either his 10-year old self, or his new partner in crime and sidekick, Rook.
MAD (Television Series)Edit
Ben appeared briefly in a snippet of the new Cartoon Network show, Mad. They were doing a Star Trek parody and the crew was watching Ben 10 on their screen.
Then, they did another parody, but this time with Benjamin Franklin. He was flying his kite when he was struck by a lightning bolt. He then gained the Omnitrix after the accident. He turned into Fourarms, Swampfire, Jetray, NRG, and Humungousaur. In the parody, all of Ben Franklin's aliens had his hair and glasses, and were miscoloured. He spent the longest time as Humungousaur so he could defeat the British people who were dissing the USA, and smash down a junior high school bully's house as revenge for calling Ben Franklin, "Fatty Magee".
He appears in the Generator Rex episode Heroes United in order to help Rex deal with a "common threat" which was the sentient Alpha nanite. His appearance changed based the series art.