To be fair, it was a two-parter in the sixth arc. You wouldn't have caught it if you weren't paying close attention.>>86886857
During his preteen years in the Null Void, Kevin discovered a Plumber splinter group called the Rooters, and they were astounded by his unnatural power-absorption ability. They brought him into their facility where they were trying to give alien powers to humans, and Kevin's power was the missing piece to their puzzle.
Telling the Rooters about his encounters with Ben Tennyson, they realize that the all-powerful Omnitrix is in the hands of an irresponsible kid. These Plumber agents, knowing what they know about the device (shit like Alien X the reality-warper or the highly destructive Waybig within Ben's reach), decide that they gotta get to work with their human-alien hybrids to stop Tennyson before he goes out of control.
Having a shared cause, Kevin agrees to be used as a funnel for transferring alien energy to human beings. Among these hybrids are the Amalgam Kids, the team of alien-human teenagers that Ben encounters in Alien Force; and among the alien power sources is Argit.
After some training, Kevin and the Amalgam Kids are given fake memories so that they can get close to Ben without compromising their mission. They are sent back to Earth, and Kevin eventually meets Ben during their teenage years, acting all goody-goody to get on Tennyson's side.
TL;DR: It turns out that the reason Kevin was all goody-goody in Alien Force is because of a Plumber splinter group in the Null Void.