Kevin Levin (Classic)/Ben 10

===Background===
Kevin Levin was born in a poor household with an unknown father and his mother. Kevin's real father was abusive and mistreated Kevin to the point where he ran away from home. During his runaway, he stole a bike and wore its padlock around his neck as a souvenir of his first ever crime.Kevin 11 Afterwards, he made his way towards New York and made his home in an abandoned subway.
===Ben 10===
Kevin was first encountered by Ben in an arcade, where, after befriending him, he was assaulted by bullies and saved by XLR8. The two attempted to steal a video game from a warehouse and got caught by SWAT agents, forcing Ben to reveal the Omnitrix to him in order to escape. The two subsequently formed a partnership to use their respective powers for personal gain; however, when Kevin rigged two trains (one carrying money and one carrying people) to collide, not caring that the crash would most likely result in the deaths of several hundred people, Ben suffered a crisis of conscience and broke off their partnership. Ben transformed into Heatblast instead of transforming into Four Arms, to stop Kevin, but to even the odds, Kevin grabbed onto Heatblast and absorbed his DNA, transforming him into an incomplete version of Heatblast. When Heatblast foiled Kevin's plans and escaped, Kevin left to use his powers to get revenge on the bullies that tormented him. However, his powers disappeared, and he transformed back into to his normal form.
When Four Arms showed up to stop Kevin, Kevin absorbed Four Arms' DNA, transformed him into an incomplete version of Four Arms. As Four Arms and Kevin fought, Four Arms won and, after reverting back into Ben, offered Kevin a new home and a chance to be a good guy. Kevin agreed to reform and go with Ben, but revealed he lied and tried to take the Omnitrix off of Ben. Feedback from it transformed Kevin back to normal and appeared to have removed the powers he absorbed. He escaped and, at the end of the episode, it was revealed that he still had Heatblast's powers.
Kevin served as the main antagonist of the second season. His ability to project heat at the end of Kevin 11 was 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 was poorer than that of the Omnitrix itself, the energy suppressed his real form and forced 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 an 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 failed altogether, causing him to mutate into what Vilgax described as a "misshapen, chaotic amalgam of creatures from the Omnitrix", which Kevin called Kevin 11. Because of this, Kevin 11 fought Four Arms and, as he was about to kill him, Lt. Steel from the Special Alien Containment Team (SACT) shot him into the water, defeating him. However, at the end of the episode, he was revealed to be alive.
In Grudge Match, Kevin 11 attacked Ben as Diamondhead 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 invulnerability + 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 had 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. Kevin defeated Technorg and took control of the Megacruiser.Back With a Vengeance
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.
A dream version of Kevin appeared in Perfect Day as a hall monitor of a dream version of Ben's school. In the dream, Kevin was normal but then changed into Kevin 11 and fought Ben. When Ben took control of the dream, he was then able to defeat the dream Kevin 11.
 
In ...Nor Iron Bars a Cage, it was revealed that back during Kevin 11's time in the Null Void Incarcecon, 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.