Ben 10 characters
The two attempt to steal video games from a warehouse and get caught by the police, forcing Ben to reveal his powers to him in order to escape. Eventually, Ben breaks off their partnership when Kevin rigs two trains to collide, not caring that hundreds of innocent people would be killed. He then absorbed the abilities of Heatblast and Fourarms, using the former form's powers to get revenge on those who had wronged him. He absorbed the latter form's powers during his second battle with Ben to even the odds. After trying to take the Omnitrix from Ben, the Omnitrix emitted a feedback pulse to prevent Kevin from taking it, seemingly neutralizing his power. Kevin, however, exhibits the ability to create flame at the end of the episode.
Kevin serves as the main antagonist of the second season. His ability to project heat at the end of "Kevin 11" is a result of the Omnitrix's feedback pulse: the amount of energy it emitted allowed him to fully absorb its powers, giving him access to every alien form Ben had at the time. However, because Kevin's ability to regulate this power is poorer than that of the Omnitrix itself, the energy suppresses his human 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 crime spree in Los Angeles, 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 permanently mutate into what Vilgax would describe as a "misshapen, chaotic amalgam of creatures from the Omnitrix."
After his mutation, Kevin gloats that he has all of Ben's powers plus his own, making him "Kevin 11" (the name Kevin is credited under in the ending sequence of the show). Despite Kevin's claim, Ben notes that Kevin's powers are only one-tenth as strong, (although this assessment may not be accurate) and Kevin never exhibits his original powers in this form (the same is true for Ghostfreak, Upgrade (though this is debatable when Kevin used XLR8's tail to slam a tray of Megacruiser food into Technorg's face), Grey Matter, or Wildmutt. Kevin's form also limits his ability to use the special powers of his alien forms. Being a random fusion of aliens, Kevin only has access to the powers most prevalent in his form. In addition to these limitations, Kevin seems unable to use the powers of any aliens Ben has gained since their original encounter.
During his third appearance in "Grudge Match", Kevin attacks Ben to get revenge, blaming his condition on Ben. As the two battle, they are abducted and forced to fight in Slix Vigma's gladiator games. During their battles, Ben helps Kevin to realize the advantages of his mutated form. Because Kevin 11 is an amalgam of aliens, he can mix and match their powers to make up for their reduced potency. Although forced to work together, Kevin's only goal was killing Ben. He is transported to a different galaxy at the end of the episode while locked in combat with Technorg, whom he managed to kill. He then took the ship as his own.
Kevin returns in the episode "Back With a Vengeance", where he teams up with Vilgax to defeat Ben, and eventually manages to remove the Omnitrix. However, he betrays Vilgax later in an attempt to leave both 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.
In "Ken 10", Kevin returns after at least thirty years of imprisonment in the Null Void (though he has been sent in and out several times prior to the episode), having been set free by his son, Devlin Levin. Devlin was born after one of these escapes in which Kevin fled to a colony on Saturn, where he started a family before being captured again. Kevin has now fully mastered his powers and can return to his human form. In his human form, he can also utilize the powers he has absorbed to a limited extent, such as XLR8's speed, without having to transform into that alien, and is able to simply shrug off an intense beating from Way Big. His mutated form, though still an amalgam, has changed considerably from his continued absorption of alien DNA, leading to his new title of "Kevin 11,000". Eager for revenge and having not so much as a care for his son, despite Devlin's role in releasing him, Kevin proceeds to beat on Ben and his son, Ken. Ironically, it is Devlin who stops him, trapping his father back in the Null Void.
In the video game Ben 10: Protector of Earth, Kevin as a mini-boss Ben fights at the end of Lumber Mill. He reappears when a Null Void portal is opened up and frees him. While Ben deals with several monsters also unleashed as a result, Kevin kidnaps Max and takes him to the Lumber Mill. Once beaten, Kevin is sucked back into the Null Void.
Kevin reappears in Ben 10: Alien Force as an ally of Ben and Gwen. Kevin's powers operate differently from the way they did in the original series. He has a normal human form, and his absorbing abilities now allow him to absorb the properties of matter as opposed to draining energy, allowing Kevin to cover himself and others with a layer of any solid substance he is touching (similar to Marvel's Absorbing Man). He has a green sports car which he takes care of obsessively. Through the years, Kevin has gained a considerable amount of knowledge about aliens, possibly through his brokering of alien technology to various aliens and the Forever Knights, this and possibly because he spent a short amount of time in space. He also seems to know a lot about humans with an alien heritage, hinting that his powers may have come from alien ancestry, and that his guess about Gwen's powers being of alien origin was proven correct. As revealed in the episode "The Gauntlet", Kevin, as mentioned by JT, is now reputed as "the toughest kid in town", indicating that he has taken up residence in Ben's hometown.