Kevin returns in Alien Force, where he is eventually released from the Null Void (he stated in Darkstar Rising, he did his time for his crimes) returned to normal, and has grown into a tall and muscular young man who has rejoined society and reconciled with his family. He has also become an alien weapons dealer, giving him several contacts with criminals such as Argit, Vulkanus and Sunder as well as a considerable knowledge in alien tech. He has also learned about his ability to absorb solid matter instead of energy (taught how to do this by Kwarrel).
In Ben 10 Returns: Part 1, Kevin attempted to organize a deal between the Forever Knights and the DNAliens in order to sell them illegal Level 5 weapons. However, the deal is interrupted by Ben, Magister Labrid, and Gwen. Trying to get revenge for the past humiliating defeats, Kevin fought Ben, but is easily defeated while the other villains escaped without paying him. Labrid then forced Kevin to help them in finding back the weapon, offering him to avoid problems with the Plumbers then. Though Kevin mainly accepted in order to get his pay, Magister Labrid later sacrificed himself in order to save him from a defective weapon's explosion. This, plus Kevin's interest for Gwen, his promise to Magister Labrid, and his desire to start a better life and become a Plumber like his father, led him to agree to teaming up with Ben and Gwen against the High Breed invasion.
Though he initially still felt a little rancorous toward Ben, Kevin slowly reconciled with him, and he proved to be a valuable ally as well as a good friend. Despite his reformation, Kevin still tends to engage in unethical activities: he is not above stealing gold and jewelry from the Forever Knights and Plumbers equipment from Manny and Helen when presented with the opportunity.
In Darkstar Rising, Kevin reveals that his father was a Plumber. When he was little, his mother told Kevin stories about what his father did.
This is a big gap in the original series because in Ben 10, he said that his parents both left him because of his Osmosian powers; this was explained later on in the series as an exaggeration on Kevin's part: Kevin's mother remarried, and in retaliation Kevin destroyed their home by accidentally absorbing energy, which lead to his stepfather causing a huge strain between Kevin and his mother, ultimately making Kevin think they hated him (partly due of the side effect of absorbing energy) and ran away. At the end of the episode, Kevin, along with Gwen and Ben became official Plumbers, after which Kevin leaves to tell his mother about his membership.
In Vengeance Of Vilgax, Kevin was subjected to another feedback pulse from the Omnitrix after Ben tampered with it. Because of this, he was unable to revert to his normal form, instead becoming a living colossus composed of different matters. But he learned to use his powers in various new ways. Throughout the season, he was shown to hate what he had become, to the extent of breaking things off with Gwen (much to her dismay), wearing an I.D. Mask in public to appear human, and considering himself a monster. He also discovered that his father was murdered by Ragnarok, and became bent on vengeance, so much that he eventually left Ragnarok to his death when he had the chance to save him.
In The Final Battle, Vilgax attempted to separate Ben from his friends, Kevin was kidnapped by Albedo in an attempt to split them up. He was also asked by Ben to back him up when taking on Vilgax, which he did. After the Omnitrix was destroyed, he reverted to normal, revealing that it was the Omnitrix that kept him that way.
During the course of the series Kevin develops a romantic relationship with Gwen, even kissing at one time.