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 the police, 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 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. When Heatblast foils Kevin's plans and escapes, Kevin leaves to use his powers to get revenge on the bullies that tormented him. However, his powers disappear and he reverts to his normal form.
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 fought, Four Arms wins and offers Kevin a new home and a chance to be a good guy. Kevin agrees to reform and go with Ben, but reveals he lied and tries to take the Omnitrix off of Ben. Feedback from it changes Kevin back to normal and appears to have removed the powers he absorbed. He escapes and at the end of the episode it is revealed that 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 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 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, defeating him; however, 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' strength and arm count, 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 claws, and XLR8's speed and prehensile tail as well as 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 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 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 enhanced 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.
A dream version of Kevin appears 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 (which, with its Ripjaws' mouth and teeth more pronounced than usual, appeared uglier than it used to be) and fought Ben. When Ben took control of the dream, he was then able to defeat the dream Kevin 11.