Kevin Levin (Classic)/Alien Force

Sometime prior to Alien Force, Kevin was eventually released from the Null Void (he stated in Darkstar Rising, he did his time for his crimes) returned to normal. He grew into a tall and muscular young man who rejoined society and reconciled with his family. He also became 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.
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 was interrupted by Ben, Magister Labrid, and Gwen. Trying to get revenge for his past humiliating defeats, Kevin fought Ben, but was easily defeated by the latter while the DNAliens escaped without paying him. Labrid then forced Kevin to help them in finding the weapon, offering him to avoid problems with the Plumbers then. Though Kevin mainly accepted in order to get his pay, in Ben 10 Returns: Part 2, Magister Labrid sacrificed himself in order to save him from a Laser Lance's explosion. This, plus Kevin's interest in 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 Highbreed 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 tended to engage in unethical activities: he was not above stealing gold and jewelry from the Forever Knights and Plumber equipment from Manny Armstrong and Helen Wheels when presented with the opportunity. He also had a rival in the form of Michael Morningstar.
During the course of the series, he developed a romantic relationship with Gwen. They even almost kissed in the episode Darkstar Rising.
In Darkstar Rising, Kevin revealed that his father was a Plumber. When he was little, his mother told Kevin stories about what his father did. 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 combination of different materials. 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 wearing an ID Mask in public to appear human and considering himself a monster. 
In Trade-Off!, Kevin teamed up with Darkstar to turn back to normal. He succeeded, but lost his powers, making him useless in battle to the team. Gwen found out what he did and went after Darkstar. Darkstar then absorbed all of her power, and Ben's while he was Swampfire. Kevin realized the only way to save them was to stop Darkstar. After stopping Darkstar, he got his mutation back and the Tennyson cousins were saved.
In Vendetta, Kevin 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.