When they first met, Kevin was a renegade eleven-year old, who knew Gwen as Ben's smart-mouthed cousin. In Kevin 11, Gwen says she does not like Kevin, and that he is just trouble, and Gwen and Kevin do not show any interest for each other. He later had a 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. Gwen tries shooting Kevin, but he grabs the blast, making it slant backwards and hit Gwen, causing her to scream. Ultimately he gets trapped in the Null Void.
In Ben 10: Alien Force, there was instant chemistry. Gwen's previous pity and admiration of Kevin turned to infatuation then love. In the first two seasons we see the two falling for each other. The problems they have or issues did get in the way because both of them cared about each other.
In What Are Little Girls Made Of?, Kevin did not want Gwen to go to Anodyne to learn how to fully master all of her Anodite and magical powers and abilities, leaving him and Ben behind. Gwen kissed Kevin on the cheek revealing she likes Kevin's sweet and sensitive side. The two work well on the battlefield. Whenever Gwen was knocked down in a fight Kevin would automatically be by her side helping her up and so 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.
They danced together in Save the Last Dance. Kevin gets mutated for the second time when he and Ben try to hack the Omnitrix. Fluctuating between bouts of uncontrollable aggression and severe depression and considering himself a monster, Kevin tries to break off things between himself and Gwen. He also gave Gwen a "keep me in your memory" locket telling her to remember his old self before the accident in Fool's Gold.
His appearance didn't bother Gwen but it proves just how much he cares about it. As seen in the episode In Charm's Way, Kevin still strives hard, though he vents his frustration in the wrong way (on Gwen) thus falling for Charmcaster which made conflict. In the end, Ben informs Kevin that Gwen has been working day and night, and going through every spell book to find a cure for him, leaving him to regret getting mad at her. However, he tries to make it up to her by saving her from Darkstar in Trade-Off leaving him from human form into his mutation. Gwen though still cares for him just as much such as in Time Heals and Vendetta where she would go to desperate measures to make things up.
In The Final Battle: Part 2, after Kevin returns to normal, they share an on-screen kiss.
However, things got settled and changed in the episode Too Hot to Handle after Gwen got hit by P'andor's radioactive blast, he quickly held her, helping her get up, and told her not to ever do that to him again. Later on the episode, he made Gwen leave the power plant because he was worried about her.
In Fused, after Gwen casts the teleportation spell, she nearly fainted and landed on Kevin's lap. There was some obvious eye contact too.
In the episode Hero Time, it is shown that Gwen does not approve Kevin falling for other girls instead of her. For example, Gwen threateningly told Jennifer Nocturne that she would "peel her like a grape" after seeing Kevin quite happy of Jennifer thanking him for saving her life.
In Ultimate Aggregor, Kevin kissed Gwen on the cheek after she took him to one side to talk to him. When Kevin reveals the insanity of Osmosians when they absorb energy, Gwen discovers that absorbing too much energy was the reason for Kevin's beserk and criminal behavior when he was an eleven-year-old child.
In the episode Map of Infinity, Gwen is defeated by Aggregor and Kevin helps her, soon Kevin is defeated too, when the two are up, they embrace, Ben, Gwen and Kevin are in search of the Map of Infinity, where Gwen collapses under a gate that Ben and Kevin are holding. The two despairing, just Aggregor takes the first fragment of the map, and Kevin saves Gwen.
In Perplexahedron, Kevin says he wants to take care of her in front of Swampfire and admits he likes her. And after Swampfire freed Gwen from the ice, she and Kevin kiss.
In The Enemy of My Enemy, Gwen is determined to reach Kevin's human heart. She stops him from killing Ben, and yells at him for brutally injuring Argit, telling him that is enough, horrified that he would do such a thing. Kevin attacks her and is about to kill her as well with fire, but stops himself at the last second. He tells Gwen that the only reason she is alive is because of what she used to mean to him. He than phases through the ceiling into space, leaving Gwen with tears in her eyes,showing that he still cares for her.
In Absolute Power: Part 1 and Absolute Power: Part 2, Gwen is more determined than ever to restore Kevin to his normal self, attempting to talk him out of his rage and insanity several times to no avail. Kevin reveals that he is avoiding her for he doesn't want her to get hurt because of him. But he loses control and his need for energy overwhelmed him in the end. Kevin is restored to his normal self with the help of Cooper, who is given a kiss on the cheek from Gwen as thanks, only for Kevin to sarcastically reply in return "Hey,since your kissing people..." while pointing at his face. They share a romantic kiss as a result, celebrating in the return of the normal Kevin Levin.
In Girl Trouble, Gwen is annoyed when Sunny tries to flirt with Kevin. Gwen angrily blurts out, "I'm right here Sunny!"
Gwen had a small crush on Winston in the episode The Creature from Beyond which led Kevin to being jealous and very protective of Gwen and when the Lucubra went into Gwen's mind he was very reculant of Gwen fighting with them and said: "Listen Gwen I can't... I mean... we can't afford to lose you... the team, you know" which really touched her and made her speechless.
At the beginning of It's Not Easy Being Gwen Kevin calls Gwen and asks her if they can meet for lunch, before Gwen could answer him a tiny fire starts due a spell because of which, she says that she will call him soon. Later Kevin shows up and tells her that she didn't call him back; she tells him that she does not have time to have lunch and is shocked seeing Kevin's shirt ripped and he replies that he found Dr. Animo. Kevin and Gwen talk on their way to her house. In her room, she tells him that even if he does not go to school she does not want him to go and fight their enemies. Kevin was shocked that she knows about him not going to school and Gwen reveals that she already known that since he was stuck in the Null Void and that he missed a lot of school years. She says that she knows that he is very smart and shows that Gwen cares about Kevin getting an education.The two are about to kiss but are interrupted by Ben who tells Kevin to hurry up as he fights off more mutant frogs. Kevin tells Gwen she's too busy to assist and jumps out the window to fight. She gives him a charm and tells him to use it if it gets to dangerous, he says ok, but Gwen says that if he use the charm and it does not work, she will never see him again. He tells her that he will see her tonight (meaning Gwen and Kevin are going on a date). Then he says that they will talk about him getting his GED. Gwen later arrives home while her mom is getting dinner ready. She tells Gwen that her father is getting dinner and that Kevin called her and tells her that Ben and Kevin will be joining them.
Kevin thinks Charmcaster is hot, and was clearly attracted to her Caroline form. Charmcaster kissed Kevin on the lips which almost created a conflict between him and Gwen. It should be noted that Charmcaster and Kevin shared a kiss long before the first on-screen kiss between Gwen and Kevin.
After this incident, Kevin and Charmcaster are mostly enemies, with Kevin disliking Charmcaster until he learns just how similar they are. He initially attempts to brush this empathy off, but following the ordeal with the Rooters which upturns his whole life, he is more visibly open in his pity at Charmcaster's situation and how much he relates to it. When Gwen traps Charmcaster in her own bag and keeps her under the goal of becoming friends with her, Kevin seems relieved, knowing first-hand how Gwen can help change a person.
Jennifer Nocturne was a 19 year old teenage actress Kevin admired, not just liking her skills but also herself. He introduced himself just before she took Ben to the party, but she paid little attention saying, "Oh of course you are." Later when Nocturne was kidnapped and saved by Kevin and Gwen, Nocturne asked Kevin to be her boyfriend. When Gwen got jealous of her and said angrily "I will peel you like a grape!".