The Omnitrix/Ultimatrix symbol is located on his chest
He is a humanoid, plant-like alien that has an overall green and black colored body mostly with a flame-patterned head and root-like feet seemingly holding rocks. His shoulders and head also have red petals. He is taller than an average human and has a distincct rotten smell that worsens with heat. He has a very stuffy and nasally voice because he doesn't have a nose.
His first appearance is similar to that of his Alien Force/Ultimate Alien design, with the exception of having a different shaped head and having more of root-like feet and now has five fingers.
His second look, a matured/blossomed form, is a major difference from his first look, still having a plant/fire-like appearance, but being more muscular. He now has light green skin and his face is now yellow and having a more visible chin. His claws are now black with yellow claws. He now has ten red horns, three being located on his arms, one on each foot, and one on his forehead. He has a part of a robe around his neck and has red spots on his elbows and knees.
Flame Projection: He has the ability to ignite the methane emitted from his palms to produce flames. This ability let's him do different feats.
Flight: By pointing his palms to the ground and shooting fire out he can have a limited flight ability.
Chlorokinesis: He has the ability to control any biological plant life, including himself, to do various tasks.
Gas Projection: He can spray a methane gas that can knock a person out. It can even knock out an Incursean without even spraying the gas.
Plant Control: He can create seed bombs that don't explode but grow into a flurry of plants/vines under his control. When he blossomed it was revealed that Swampfire plant control abilities have been extended to be able to control the mutant pumpkins from the Anur System, he was also able to control/grow plants with just a raise of his hands, as seen in Charmed, I'm Sure.
Elasticity: He can also manipulate parts of his body,like stretch himself to catch far away items, to even burrowing underground to fast travel from one location to the next.
Regeneration: Due to his plant-like composition he can regenerate at a very quick pace, this ability also allows him to walk through Iron Bars as seen in X = Ben + 2. .
Enhanced Everything: He is strong enough to take a Highbreed weather tower down and take on Vilgax with his bare fists, he also is shown to have enhanced speed as seen when he was seen running toward Vilgax as the scenery was blurry as if he were running really fast. He also has enhanced agility. As seen in Charmed, I'm Sure, in his blossomed/matured form he seems to have his strength and durability increased massively.
He, even with the ability to control fire, has a slight disadvantage because he is more vulnerable than Heatblast to ice because he can be frozen for immense periods of time but he can unthaw by igniting the methane in his hands. This was shown when the DNAliens froze Swampfire and Albedo turned into Big Chill and froze him.
He can be cut up to the point in which he can't regenerate, as seen when Goop's Anti-Gravity Projector cut Swampfire into pieces.
According to Ben, he can't control weeds.
His swampy stench makes it hard for him to hide from anyone with a nose.