When they first met, Kevin, was a renegade 11-year old, who knew Gwen as Ben's smart-mouthed cousin. Gwen distrusts Kevin at their first meeting and says that she does not like Kevin and that he is just trouble. Gwen and Kevin do not show any interest for each other. He later had a vengeance against Ben and almost burned 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. They barely come into contact for the remainder of the series.
Five years prior to Omniverse
11 year old Gwen had began to develop feelings for Kevin during the times that they have met.
In one of the Omniverse flashbacks presented in Mud Is Thicker Than Water, Cousin Lucy is asking Gwen about what Kevin is like. Gwen tells her that Kevin is a guy who thinks that he is cool because he is a bad guy. Lucy laughs and then teases about Gwen liking Kevin but Gwen rejects the theory.
In an Omniverse flashback in Weapon XI: Part 2, Gwen encounters Kevin (who was with the Amalgam Kids at the time). During the fight between Ben and Kevin, Ben transforms into Eye Guy and manages to knock and pin Kevin down to the ground, but Kevin once again absorbs DNA from Omnitrix and undergoes another mutation. Kevin went insane and started to blast at Ben, Gwen, Max, and even Amalgam Kids. Kevin overpowers all of them but is suddenly taken down by Argit who shoots his quills at Kevin's head, knocking Kevin out and he transforms back to normal. After everyone is freed from the vines, Gwen walks other to and looks over Kevin. Gwen then pulls the quills off of his head and asks Kevin if he was okay. Kevin greets her weakly and smoke fills the area and Kevin, Argit, and the Amalgam Kids return back into the Null Void.
In From Hedorium to Eternity, Ben and Gwen both encounter Kevin again underground and she and Ben (along with their cousin Lucy and friends Cooper and Jonesy) join forces with Kevin to stop Zs'Skayr and his Ectonurite minions from taking over the Earth. Kevin teases Cooper about being Gwen's boyfriend, and says that he thinks he is a better guy, leaving Gwen raged and walks away. Gwen is surprised that Kevin has been helping Zs'Skayr and other Ectonurites are inside the bodies and controlling the grown ups. Due to the collective efforts of all the kids, everyone was saved when suddenly the Null Void portal starts to pull Kevin. Ben, Gwen, Lucy, Jonesy and Cooper help to pull Kevin in order to prevent him from being pulled into the Null Void portal, but they failed to do so and Kevin gets pulled back into the Null Void, much to Gwen's sadness. Later, outside Ben's house, Ben and Gwen suddenly notice Kevin climbing out of a hole. Kevin says Gwen's love was the one who brought him back from Null Void. Gwen blushes and held his hand, but Kevin is revealed to be Lucy in disguise as she transforms back into normal. Lucy and Ben tease Gwen about liking Kevin, which angers Gwen and she chases Ben and Lucy on the road.
In Alien Force, there was instant chemistry. Ben was still distrusting of Kevin, but at Gwen's decision to trust him, Ben went along with it. Slowly, the trio became good friends. Gwen's previous pity and admiration of Kevin turned to infatuation then love. In the first two seasons they start 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, which made Gwen kiss him on the cheek, meaning that Gwen likes Kevin's sweet and sensitive side. The two also work well on the battlefield. Whenever Gwen was knocked down in a fight, Kevin would automatically be by her side helping her up. 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 are rare. Instead they talk sternly but quietly in each other's faces. After some argument, both of them try their best to make up for it.
In Darkstar Rising, Gwen tries to comfort Kevin for his father's absence and his stripped privilege. The two almost kissed until a Highbreed barged in. Their relationship grows in the second season. They danced together in Save the Last Dance.
In War of the Worlds, when Kevin is threatened by a Highbreed Commander, a furious Gwen releases her dormant inner Anodite self for the first time, and almost loses her human form. However, upon hearing Kevin's confession that he can't bear to lose her, she reverts to human form and shares a warm hug with Kevin afterwards. When Kevin gets mutated for the second time 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 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 created conflict. In the end, Ben tells Kevin that Gwen's 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, sacrificing his chance of regaining his normal form. Gwen though still cares for him just as much such as in Time Heals where she'd go to desperate measures to make things up. In The Final Battle, after Kevin returns to his human form, they share an on-screen kiss, also reviving their relationship.
In Alien Swarm they are dating, but Ben doesn't get what Gwen sees in Kevin. Gwen also kissed Kevin on the cheek when he found the chips. At the beginning of the movie, he also briefly protected her from the chips. Kevin also kissed Gwen on the forehead because she was a little jealous of Kevin building Ben a car and not her.
In Ben 10: Ultimate Alien, for the first few episodes, they both don't show much interaction, and have a lot of arguments such as in Video Games and the early episodes. However, things got settled and changed in the episode Too Hot to Handle after Gwen gets hit by P'andor's blast, he quickly held her, helping her get up, and told her not to "ever do that to him like that again". Later in the episode, he made Gwen leave the power plant because he's worried about her. In Andreas' Fault, Gwen's feelings were hurt when Kevin angrily told Argit "You know I love money more than anything in the world". In Fused, after Gwen casts the 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 of Kevin falling for other girls instead of her. When Jennifer tries to ask Kevin out, Gwen threateningly told her that she would "peel her like a grape" after seeing Kevin quite happy of Jennifer thanking him for saving her life. In the episode 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 berserk and criminal behavior when he was eleven.In Map of Infinity, Gwen is defeated by Aggregor and Kevin helps her, but soon Kevin is defeated too. When the two get up, they embrace. In Perplexahedron , Kevin says he wants to take care of her in front of Swampfire and when Ben says that she can take care of herself. After Swampfire freed Gwen from the ice, she and Kevin kiss, prompting Swampfire to say in disgust "Why don't you guys get a room?!"
In The Forge of Creation, Kevin is forced to absorb the power of the Ultimatrix to stop Ultimate Aggregor from absorbing the baby Celestialsapien's power, turning him insane again. He then tells them that he is the one that no one trust and the one nobody cares about, Gwen says "That's not true!", he then tells her that no matter what he looks like he will always be a freak to everyone. He leaves the team and Gwen and Ben are then looking for him. It's shown that Gwen still loves Kevin and is horrified when Ben suggests killing him after what he was going to do to the Warden in ...Nor Iron Bars a Cage.
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, 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 then phases through the ceiling into space, leaving Gwen with tears in her eyes, showing that she knows that he still cares about her. This shows that Kevin still loves Gwen, and gives her hope that maybe Kevin's sanity can be restored.
In Absolute Power, 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. She even risks her a fraction of her Anodite and magical abilities when Kevin tries to prevent himself from his hunger of absorbing energy. But his need for energy overwhelmed him in the end. When Cooper was trying to protect Gwen telling him to "stay away from her" Kevin picks up Cooper and jealously says "Trying to make time with my girl, huh?!" and throws him up in the air and just when Kevin is about to punch him, Ben saves him. 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 you're 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 season 2, their relationship is toned down, though they have as of yet to be seen fighting, there has been little interaction couple wise between them. In The Transmogrification of Eunice, Gwen is mad when Kevin has a minor suggestive crush on Eunice. Gwen was also jealous of her cousin Sunny, who flirted with Kevin in Girl Trouble. While Kevin in turn was jealous of Winston, who Gwen had a small crush on in The Creature from Beyond. He was also very protective when the Lucubra had attack her. Later when the team is about to attack the Lucubra Kevin tells Gwen to stay away. She tells him that he can take care of herself then Kevin says "Listen, Gwen, I can't...I mean...we can't afford to lose you...the team, you know." Which had made Gwen very speechless, as she knew what Kevin was trying to say to her.
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 from doing a spell, 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 she sees that Kevin's shirt is all ripped and says that he found Dr.Animo. Kevin and Gwen talk on their way to her house and 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, they later talk in her room and says 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 he and Ben will be joining them.
In Moonstruck, Kevin and Max seem to have a lot in common such as with their cars and personalities (adding a witty comment to when they're trying to defeat a villain/bad guy). Verdona commented in "What are Little Girls Made of" that Max had a spark on his own and how Kevin reminds him of Max and how she reminds her of herself in a way (though not as many examples can be given compared to with Kevin and Max) in how she was an adventurer. Like with Kevin and Gwen in Alien Force, it seems that Verdona and Max had instant chemistry when they met each other. That and the fact that both Max and Kevin fell in love with Anodites. Kevin even borrowed Max's tuxedo in Save the Last Dance when taking Gwen to her formal.
In Prisoner Number 775 Is Missing, it seemed as if they were both understanding each other such as when Kevin commented on the way the Plumber's spaceship was bad with the security system, Gwen knew right away that he wanted one to which he agreed. She responded "Maybe for your birthday" patting him on the shoulder which made both of them smile. There was also the case when Rath was climbing up the stolen plumber's spaceship where he commented that Rath is the only alien that Ben transforms into that makes him want to sit down with a bag of popcorn and watch, to which Gwen asks if that's really appropriate for right now. When Kevin looks kind of uneasy, she responds "Okay, me too" to which he smiles. When the team finds a knocked out police man, they agreed that it was OK to beg to stay alive to see his family again knowing how important family is. Later in the episode when Gwen says she can detect Prisoner 775 by his mana, she gives Kevin instructions on where he is "left, right, now he's left again, no wait, he's behind you now" to which Kevin keeps missing and when he's punched by Prisoner 775, he ends up on the ground in front of Gwen, with her having somewhat of a worried and sorry look on her face to which he takes his head off the ground and says with sarcasm "Thanks for the help, Gwen", but knew that she was still trying to help him in the fight.
In Ben 10: Omniverse, Gwen and Kevin's relationship has not changed that much. Gwen is first seen with Ben and Kevin chasing Zombozo through Bellwood. Shortly after fighting and capturing Zombozo, Gwen says her goodbyes to Ben because she is heading to college since she took honor classes. She then tells Ben to stay out of trouble and gets into the car with Kevin (who also reveals that he will be staying with Gwen, but near campus), then the two drive off (which leaves Ben to deal with Zombozo.)