Ben 10: Ultimate Alien focuses more on Kevin's past, including how he managed to return to normal, escape the Null Void, as well as his story prior to the original series. Kevin still has the ability to change his hands into different weapons even after he returned to normal in Alien Force.
Kevin and Ben no longer share their famous rivalry and instead have become true friends. Kevin acts as an older brother towards Ben. They care genuinely for each other. Kevin is much more open about his feelings, showing gratitude and tries to express it to both Ben and Gwen (mostly Gwen). Gwen and Kevin continue their romantic relationship. He also seemed to have a better connection towards Max.
In Too Hot to Handle, Kevin is 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 uses 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 admits 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. It comes to Kevin's notice that there is another Osmosian named Aggregor. While never explained, he shows sympathy to Aggregor because they are, somewhat of the same race and because Kevin, like Aggregor, was once an insane and violent sociopath.
In Ultimate Aggregor, Kevin shows his knack for detective work, and later warned by Paradox to remember who his friends are and that things will get worse before they get better. Kevin explains to Aggregor that the reason that he was a villain in the original series is that when Osmosians absorb energy, they become insane. Despite being true, Aggregor disagrees.
In Map of Infinity, Kevin continues to focus on stopping Ultimate Aggregor, but is easily defeated by Ultimate Aggregor, he also seemed to know about the Map of Infinity saying it was just a legend. The group go in search of the first part, Kevin wears a Plumbers' Suit, unlike any other Plumbers' Suits, that Kevin had a little white glove and a circle over the glove (which gives Ben access to the Ultimatrix), probably this glove is for Kevin to use his powers. Kevin proved to be very strong when a giant hammer crushes the group on a wall, he survived and managed to save the group and remove the hammer from the wall.
In Deep, we see Kevin using a Plumbers' Suit with built-in electrical weapons. His helmet is destroyed, but Gwen uses her powers to make Kevin absorb the matter of the Rust Bucket 3, saving him from being crushed by the pressure. However, Kevin still can not breathe, until Blowfish acts as a helmet for him to breathe, in the core Kevin takes the Blowfish from his head (he did not need him since there was breathable air in the core), Kevin also finds and tries to fight Ultimate Aggregor, unfortunately he is easily defeated by Ultimate Aggregor's electric attack, vulnerable because of the matter he absorbed was conductive, Ultimate Aggregor advised that next time, Kevin should try absorbing something non-conductive, and escapes. Kevin leaves the Blowfish in Piscciss and is comforted by Gwen at the end, as he seemed to miss Blowfish.
In Perplexahedron, when Ben and Kevin get separated from Gwen, Ben says that Gwen can take care of herself and Kevin replies, "That's why I like her." Kevin then talks about his feelings to Ben and how he owes it to Ben and Gwen changing his life. After finding Gwen, Kevin tries to say that he likes her. Then Swampfire tells her what Kevin's trying to say, and Gwen says, "I know that already." and they kiss.In The Forge of Creation, as a last resort to stop Ultimate Aggregor from succeeding, Kevin absorbs the powers and abilities from the Ultimatrix's unlocked aliens, becoming Ultimate Kevin, powerful enough to defeat Ultimate Aggregor. Unfortunately, this also causes him to become insane once again. His old desire for power also returns as he absorbs Ultimate Aggregor's powers as well. However, Kevin's good heart appears to not be taken over completely, as he accepted 10-year-old Ben's harsh criticism before sparing Aggregor's life, though he declares he will not be so merciful tomorrow.
In ...Nor Iron Bars a Cage, it is revealed that back during Kevin 11's time in the Null Void, 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. Out to avenge his 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 is truly gone, Ben, much to Gwen's horror, suggests 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 Rust Bucket 3 in order to get Argit, but was forced to retreat due to his car being thrown off the plane. He then attacked the Plumber's Academy and easily took care of the rookie Plumbers. He then attacked and thought he killed Argit (due to Argit closing down his pulse). During his attempt to kill Argit, he nearly killed Gwen, thinking she betrayed him. However, he restrains himself and spares her life, saying: "The only reason you're alive is because of what you used to mean to me." before departing, making is clear that a part of Ultimate Kevin still cares about her and Gwen remaining adamant that they can still bring Ultimate Kevin back to sanity without killing him.
In Absolute Power, in hopes of trying to save Kevin, Gwen inquires the aid of Darkstar. However, Ben is very furious with the idea. This is Gwen's last hope of trying to bring Kevin back. Gwen tries to talk to Ultimate Kevin, who tells her he's been avoiding her because he can't control himself, implying that he doesn't want to hurt her; but he gains the desire to absorb Gwen's powers.
After Ultimate Echo Echo defeated Ultimate Kevin, Cooper finished the machine that would siphon all of the energies that Kevin had absorbed and subsequently cure him with it. Darkstar then betrays the team by using the piece of the Dominus Librium to transfer all said energies into himself, which backfires when Ben; anticipating such a move, activates a switch to return all of the energy back to where they belong, Kevin then knocks him out with a sucker punch. Kevin thanks Cooper. He is then given a passionate kiss on the lips by Gwen (much to the disappointment of Cooper) and gets into an argument with Ben about their previous fight, to which he is relieved that everything turned out alright, after which Gwen asks for their help in explaining to her parents about the damage done to their home, to which Kevin and Ben ditch by voting to go to Mr. Smoothies instead, with Kevin buying.
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 Monok, 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. After Julie returned the Sentinel's missing eye, he rephrases Ben's question with the word "in" instead of "on" and has Ben talk to Julie and the two became a couple again.