Kevin Levin (Classic)/Ultimate Alien

In Ultimate Alien, Kevin and Ben no longer shared their famous rivalry and instead had become true friends. Kevin acted as an older brother towards Ben, and they cared genuinely for each other. Kevin was much more open about his feelings, showing gratitude and tried to express it to both Ben and Gwen.
In Too Hot to Handle, Kevin was shown to be able to repair objects in the current matter he has absorbed by releasing a liquid version of that substance and then hardening it again. He can also manipulate whatever matter he is touching by absorbing it and then morphing it into any shape or form he wants. He used both the abilities when he fixed the broken seal of P'andor's suit, morphed it around his body to trap him, and fused the arms together as makeshift handcuffs.
In Fused, Kevin admitted he was selfish and a coward when he first met Ben. He also risked his life to save Ben from being taken over by Ra'ad.
In Ultimate Aggregor, Kevin showed a knack for detective work, and was later warned by Paradox to remember who his friends are and that things would get worse before they get better. Kevin explained to Aggregor that the reason that he was a villain in the past is that when Osmosians absorb energy, they become insane. Despite it being true, Aggregor disagreed. Throughout the episode, Kevin was more focused on stopping Aggregor than usual, stating that he felt responsible due to both of them being Osmosians.
In Map of Infinity, Kevin continued to focus on stopping Ultimate Aggregor, but was easily defeated by him. He also seemed to know about the Map of Infinity, saying it was just a legend. When the group went in search of the first part, Kevin proved to be very strong when a giant hammer crushed the group against a wall, as he managed to save the group and remove the hammer from the wall.
In Deep, the team traveled to Piscciss to find the second piece of the Map of Infinity. The helmet of Kevin's Plumber suit was destroyed, but Gwen used her powers to make Kevin absorb the matter of the Rustbucket 3, saving him from being crushed by the pressure. However, Kevin still could not breathe, until an unnamed blowfish acted as a helmet for him to breathe. When the team reached the planet's core, Kevin removed the blowfish from his head (he did not need him since there was breathable air in the core). Kevin later tried to fight Ultimate Aggregor, but was easily defeated by Ultimate Aggregor's electric attack, as he was vulnerable because the matter he absorbed was conductive. Ultimate Aggregor advised that next time, Kevin should try absorbing something non-conductive, and escaped. At the end of the episode, Kevin left the blowfish in Piscciss and was comforted by Gwen at the end, as he seemed to miss the fish.
In Perplexahedron, when Ben and Kevin were separated from Gwen, Ben reassured Kevin by saying that Gwen could take care of herself. Kevin said he knew she could take care of herself, but he wanted to take care of her anyway. During this time, the two also talked about their feelings. Ben told Kevin that he considered him the big brother he never had. Kevin then said that he was grateful that Ben and Gwen gave him another chance, and that he owed them for changing his life. Ben and Kevin later found Gwen, and after Swampfire freed Gwen from the ice, she and Kevin kissed, prompting Swampfire to say in disgust "Why don't you guys get a room?!"
thumb|Ultimate Kevin 
In The Forge of Creation, in order to stop Ultimate Aggregor from absorbing the powers of a baby Celestialsapien, Kevin had no choice but to absorb the Ultimatrix's powers and mutate again. This form, named Ultimate Kevin, easily defeated Ultimate Aggregor and drained his powers, reverting him back into Aggregor. After being scolded by Young Ben for his selfishness when he attempted to absorb the baby Celestialsapien, Ultimate Kevin flew away from the Forge of Creation.
In ...Nor Iron Bars a Cage, to avenge Kwarrel's death, even though it went against all that Kwarrel had taught him, Ultimate Kevin purposely returned to the Null Void to kill Morgg. He would have succeeded if Gwen hadn't saved Morgg. Believing that Kevin was truly gone, Ben, much to Gwen's horror, suggested that Ultimate Kevin must be put down.
In The Enemy of My Enemy, Ultimate Kevin went after Argit for all the times he double-crossed him. He first attacked him and sent him towards Ben and Gwen. He later attacked Rustbucket 3 in order to get Argit, but was forced to retreat due to his car being thrown out of the ship. He traveled to the Plumbers' Academy and easily took care of the Plumber cadets. He then attacked and thought he killed Argit (due to Argit shutting off his pulse). During his attempt to kill Argit, he nearly killed Gwen, thinking she had betrayed him. However, he restrained himself and spared her life, saying: "The only reason you're alive is because of what you used to mean to me." before departing, making it clear that a part of Ultimate Kevin still cared about her and Gwen remaining adamant that they could still bring Ultimate Kevin back to sanity without killing him.
In Absolute Power: Part 1, Ben decided that he must kill Kevin to stop his rampage. Gwen disagreed, believing there could be a way to help Kevin. The argument escalated into a fight which Ben won, due to Gwen holding back. When Max, too, agreed with Ben that Kevin had to be put down, Gwen resorted to recruiting Darkstar, casting a spell to restore his looks. The two met up with Ben, and Darkstar explained that when Kevin destroyed the Dominus Librium, the ancient artifact that cured Kevin and Darkstar of their previous mutations,Trade-Off! he managed to salvage a piece and that it lost all of its power, but with enough energy channeled through it, they could tap into its powers and could cure Kevin. Ben was reluctant to follow through Darkstar's plan and was just intent on finding Kevin in order to put him down.
At Total Zone, Gwen encountered Kevin and explained that Ben was after him and that she wanted to help him, but Kevin started to lose control and attacked Gwen. He yelled at her for not staying away, with the last shred of his sanity having been his love for her (only now it was not enough to stop him). After a brief fight, Kevin began absorbing Gwen's powers and Gwen screamed in pain, while Kevin looked all the more pleased.
In Absolute Power: Part 2, Gwen escaped from Kevin's clutches. In the Rustbucket 3, Darkstar explained that since Kevin had a taste of Gwen's powers, he would want more, making Gwen the bait in his plan. They landed in Los Soledad, where Darkstar showed that he recruited Cooper to help him with turning Kevin to normal by building a machine that would tap into Darkstar's Dominus Librium piece. This would absorb Kevin's powers and all the powers he stole and return him to normal. Cooper began building the machine while Ben told Gwen that he was the only one who can stop Kevin's rampage because Professor Paradox told him and he was still reluctant to follow Gwen's plan.
Later, after a fight at Gwen's house, Gwen lured Kevin to Los Soledad. When Cooper tried to protect Gwen, telling Kevin to "stay away from her", Kevin picked up Cooper and jealously said "Trying to make time with my girl, huh?!" and threw him up in the air. Just when Kevin was about to punch him, Ben saved him as Ultimate Echo Echo and then fought Kevin. Ultimate Echo Echo was ready to deliver the final blow, but Gwen prevented him from doing so and convinced him otherwise. Kevin was then restored to his normal self with the help of Cooper, who was given a kiss on the cheek from Gwen as thanks, only for Kevin to sarcastically reply in return "Hey, since you're kissing people..." while pointing at his face. They shared a romantic kiss as a result, celebrating the return of the normal Kevin Levin.
In Eye of the Beholder, Kevin was eating a Fleen Cake that was considered the most delicious desert in the galaxy. After Ben caught up with him and the girls on Monarch, Kevin wasn't impressed with Ben blowing Julie off like a jerk and explained to him that if he never acted like a jerk, she wouldn't have dumped him.
In The Ultimate Enemy: Part 1, when Gwen was possessed by Dagon, Kevin followed her from her house all the way to the seal. As she began to break the seal, Kevin ran up to her and grabbed her arms, then covered her in the metal coating he was wearing, before pulling it back onto himself, with the exception of a metal cap over her head, to prevent Dagon from controlling her. Gwen felt guilty that Kevin had to do that, as he left himself wide open and she could have hurt him. He reassured her by saying that she could never hurt him.