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 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. Gwen tries shooting Kevin, but he grabs Gwen's hand, making it blast her hand instead.
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 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.
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.
Their relationship grows in the second season. They danced together in Save the Last Dance. When Kevin gets mutated 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. 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 (in Darkstar Rising), 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 spellbook 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.
In Ben 10: Ultimate Alien, they do not show much interaction for the first few episodes, and had a lot of arguments, such as in "Video Games". 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 and helped her get up, before telling her not to ever do that to him like that again. Later on the episode, he made Gwen leave the power plant because he's worried about her. In Fused, after Gwen casts the spell, she nearly fainted and landed on Kevin's lap, with some obvious eye-contact. In the episode Hero Time, it is shown that Gwen does not approve on Kevin falling for other girls instead of her. When Jennifer tries to ask Kevin out, Gwen taps her and says I will peel you like a grape! 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 then 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. When Ben, Gwen and Kevin learn what Aggregor is going to do, they begin the search for the Map of Infinity. When looking for the first piece, Gwen collapses due to a poison dart while Ben and Kevin are holding up a gate. Aggregor manages to take the first fragment of the map while Kevin saves Gwen. In Perplexahedron, Kevin says he wants to take care of her in front of Ben and admits he likes her. After Ben had freed Gwen, she and Kevin kiss prompting Ben to say in disgust, "Why don't you guys get a room?! This place is full of 'em!" In "The Forge of Creation" and "Nor Iron Bars A Cage", it is shown that Gwen still has not given up on Kevin, despite his current situation.
In The Enemy of My Enemy, Gwen is determined to reach Kevin. She stops him from killing Ben, and yells at him for brutally injuring Argit, telling him that it is enough, horrified that he would do such a thing. Kevin attacks her and is about to kill her, about to blast her, but he sees the tears falling down Gwen's cheeks. Kevin 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 the two-part first season finale 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 a fraction of her magical abilities when Kevin tries to prevent himself from his hunger of absorbing energy, but 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 "since you're kissing people..." while pointing at his face. Gwen walks over to Kevin, and they kiss while hugging.
In The Transmogrification of Eunice, Gwen is mad when Kevin has a minor suggestive crush on Eunice. In Girl Trouble, Gwen gets annoyed when Sunny tries to flirt with Kevin, blurting 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. When the Lucubra went into Gwen's mind, he was very reluctant of Gwen fighting with them, saying "Listen Gwen I can't... I mean... we can't afford to lose you... The team, you know," which touched her and left her speechless.
At the beginning of It's Not Easy Being Gwen, Kevin calls Gwen and asks her if they can meet for lunch, but before Gwen could answer him a tiny fire starts from her 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 doesn't have time to eat lunch. She sees that Kevin's shirt is ripped, and he 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 doesn't want him to go and fight their enemies. Kevin was shocked that she knows about him not going to school, to which she says that she already knew, since he was stuck in the Null Void and missed a lot of school years. She says that she knows that he is very smart, and shows that she cares about Kevin getting an education. The two are about to kiss but are interrupted by Ben, who tells Kevin to hurry up as Ben 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, but Gwen says that if he uses the charm and it does not work, she will never see him again. He tells her that he will see her tonight, and 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 that Ben and Kevin will be joining them.
When Gwen and the others first met Morningstar in All That Glitters, he was a charming Casanova and there was a dim spark between him and Gwen. Ben, because of his eagerness to build a team, and Gwen, because of her crush on him, automatically trusted him, but Kevin resented him, due to his crush on Gwen. After a battle against the zombies, Mike asked Gwen out on a date which made Kevin even more jealous. Later, it turned out that Mike had never liked Gwen as a person, only her innate Anodite, magical powers, and abilities. By the end of the episode, she came to her senses, regained her magical and Anodite abilities, and returned to Kevin's side which, he was happy about.
Cooper has had a crush on Gwen since the Alien Force episode Undercover. Whenever he sees Gwen he blushes, makes sweet remarks, and acts all goofy around her. Gwen doesn't reciprocate his feelings, but is flattered that he is so smitten for her.
It is seen that Gwen has a small crush on Winston from the Forever Knights. She likes his intelligence, much to Kevin's dismay and jealousy.