Gwen Tennyson and Kevin Levin

Gwen Tennyson and Kevin Levin are one of the main couples alongside Ben Tennyson and Julie Yamamoto.

Original series
When they first met, Kevin was a renegade 11 year old, who knew Gwen as Ben's smart-mouthed cousin. He later had an vengeance against Ben and almost burnt Gwen in the episode Framed. In Back With a Vengeance, Kevin holds Gwen hostage so that Ben would give up the omnitrix. Ultimately he gets trapped in the Null Void.

During the series it is evidently displayed that Gwen feels sorry for Kevin having to live the hard life such as in Perfect Day and other encounters they have. There wasn't much or barely any interaction between the two though.

Alien Force
In Ben 10: Alien Force, there was instant chemistry. Gwen's previous pity and admiration of Kevin turned to infatuation then love. In season one and two we see the two falling for each other. The problems they have nor issues did get in the way buit both of them cared about each other. In What Little Girls Are Made Of Kevin didn't want Gwen to go off leaving him and Ben. The two also work well in the battle field. Whenever Gwen's knocked down in a fight Kevin would automatically be by her side helping her up and so is otherwise. Both of them would also put their life at risk in order to protect the other. Arguments between Kevin and Gwen are quite unique. Shouts were rare but instead talking sternly but quietly in each other's face makes up for it. In Darkstar Rising Gwen tries to comfort Kevin for his father's absence and his stripped privellege. The two almost kiss before the Highbreed barged in. Their relationship grows in the second season. They danced together in Saved The Last Dance. When Kevin gets mutuated after he and Ben try to hack the Omnitrix, he tries to break off things between himself and Gwen during his bouts of self-consciousness. Kevin was quite depressed, he also gave Gwen a "keep me in your memory" locket telling her to remember his old self before the accident. His appearance didn't bother Gwen but it proves just how much he cares about it. Kevin still strives hard though he vents his frustration in the wrong way (on Gwen) thus falling for Charmcaster which made conflict. Gwen though still cares for him just as much such as in Time Heals and Vendetta where she'd go to desperate measures to make things up. In Final Battle: Part 2, after Kevin returns back to normal, they share an on-screen kiss, also remusing their relationship.

Ultimate Alien
In Ben 10: Ultimate Alien, episode Hero Time, it is seen that Gwen does not approve Kevin for falling on other girls instead of her. For example, Gwen threatningly tells Nocturne that she would peel her like a grape after seeing Kevin quite happy of Jennifer thanking him for saving her life.