In Season 1, Kevin has his Ultimate Alien look. For the most part this remains the same in Season 2 and 3, but he grows out his hair longer like he does in Omniverse.
At one point, Kevin was a villain, and because of this spent some time in the Null Void. After several altercations with Ben, he eventually reformed and joined his side.
On the 25th, Kevin went to Incarcecon with Ben and Gwen to stop the prisoners from escaping. They were successful in doing so, but Blitz got away. After wrapping things up, Kevin left with Ben and Gwen.