Throughout the course of the first series Ben and Gwen shared a love/hate relationship, which shown they are constantly fighting and arguing with each other, despite Gwen and Ben deeply cares about each other's which seen on several occasions like the episode Back with a Vengeance when Gwen went in to the null void by herself to bring Ben back, and got back the Omnitrix for him after he willingly gave it to Vilgax and Kevin in exchange for Gwen's safety, and in Secret of The Omnitrix Ben was devastated when he thought Gwen died, but after knowing that she was alive, he ran through, open field of fire and hugged her, while she hugged Ben back.
In Alien Force they ceased the fighting and it was revealed that they were each other's favorite cousin, Gwen was the one encouraged Ben to ask Julie out, and in Time Heals when Gwen saved Ben from Hex torture chamber he stated that "I've already lost you once, I'm not gonna let that happen again".
In Ultimate Alien, their relation suffered a minor setback when they both argue on how to handle Kevin, which led them to their first serious fight with Ben using Way big to defeat Gwen, however Gwen convinced Ben to try her way. They start to become closer from that point on which is seen in Greetings from Techadon when they were talking about not getting along in the past and Gwen's issue of not having a car, Ben gladly offered her to drive his car giving her the keys to drive it. In The Ultimate Sacrifice Gwen was willing to go into the Ultimatrix to save Ben and risking her humanity in order to destroy the ultimates which Ben stops her from doing. When Ben was about to sacrifice himself, he gave her a goodbye kiss on the forehead.
In Omniverse, Gwen left for college early. Both have stated that they miss each other a lot. Gwen helped Ben take down Zombozo before leaving for college, even giving a hug and telling him to take care of himself. In this series, they love getting into each other's nerves like the old times, like Ben getting an honorary degree and saying that Cousin Lucy is awesome because she loves teasing Gwen. In Ben Again, Ben has stated to young Gwen openly that he misses her, and that he finds Gwen to be cool. Ben has requested Gwen to visit for often from college. In The Frogs of War: Part 2, Gwen runs to hug Ben when she knew he was Bullfrag all along, and when Ben stated he had not drank at Mr. Smoothy's in a month, she just concludes that 'Ben Tennyson is back'.
Gwen shares the same relation type a Ben does with Max, but she seems to not show it as much as Ben does as shown when Gwen jokes around with Max's strange cooking when she states that she's surprised it didn't kill him which causes Ben to become enraged and yell at her. But she showed it though in What Are Little Girls Made Of?. When Gwen was depressed about losing Max while recalling that he once took her to his favorite fishing place.
Despite Gwen's parents disapproving her life style, especially Natalie, they never tried to stop her, they love their daughter and they are supportive her decisions no matter what which is seen in What Are Little Girls Made Of?.
According to Gwen's mother, Gwen loved playing with Sunny when they were younger. In Girl Trouble, Gwen showed to be extremely irritated by Sunny's behavior (e.g. flirting with Kevin), while Sunny developed a grudge against Gwen's due to everyone telling her to be like her cousin, when the two fought Sunny overpower Gwen until Verdona came to take Sunny, before Verdona leaving Gwen tells her not to be hard on Sunny because despite being a pain, she is still family.
When Gwen first met Verdona, they both started off on the wrong foot, battling each other. Shocked to see her manipulate mana, Verdona was soon revealed to be Gwen's grandmother. Verdona believed that Gwen had the "spark" ever since she saw her use her powers and revealed much of her alien powers and background. Verdona then tried to influence Gwen into training with her on her home planet, Anodyne, which would take about seventy-five years to master. Refusing the offer, Gwen decided to stay home. Angered, Verdona tried to force the inner Anodite out of Gwen by fighting her. Fed up, Gwen demanded her to stop and eventually came to understanding terms with her. Verdona flew home and promised to revisit every now and again.
When they were kids, Lucy loves teasing and tricking Gwen a lot. Some examples like teasing Gwen that she likes Kevin when she was younger. In Mud Is Thicker Than Water, Gwen was shocked to see Lucy who became a Plumber in a short amount of time. Lucy also disguised herself as Kevin, making Gwen confused since he went to the auto show with Rook. Lucy tricked Gwen making her angry again. Throughout this episode, due to Gwen always being tricked and teased by Lucy, Gwen suspected Lucy to be the spy in the Plumbers Headquarters stealing Plumber Tech. In the end, Grandpa Max revealed that Lucy was actually posing as Liam to find out the real traitor, leaving Gwen feeling sorry for herself and Lucy. In the end, the two of them started talking.
From Hedorium to Eternity, 11 year old Lucy disguised herself as Kevin and tricked Gwen. Due to Gwen blushing, she gets the fact that Gwen likes Kevin, leaving her irritated and started chasing each other. In this episode, they did work together into kicking Ben's soccer ball and scoring a goal, leading their teamwork to a success.
Emily has been Gwen's best friend since kindergarten. They did everything together up until Gwen was halfway through Freshman year (start of Alien Force). Emily was able to understand Gwen's stressful life therefore being Gwen's personal hero when it comes to food. Because of her not being a hero like Gwen, she was unable to join Gwen in her Plumber activity. When Gwen had to join Ben and Kevin to fight Animo, Gwen was not willing to leave her friend, but she knew she had a responsibility.
Due to the relationship with Ben, Julie and Gwen have developed a very close friendship, doing girlish activities together like shopping or doing homework together. Usually when Ben and Julie have relationship problems, Gwen usually comes in Julie's defense knowing how obnoxious Ben can be. When Ben doesn't help Julie, Gwen usually is the one to help her. On one occasion, Julie was in danger because the Vreedle Brothers were given a task to take ship for a prize in return. Ship merged with Julie to protect her, but it wasn't enough. Gwen risked her safety to save Julie and fight them off together.
Not much of a bond has been formed with her cousin's new teammate. However, in Many Happy Returns, it does appear that the two get along perfectly fine. At first she spoke about her college studies with him and even spoke about her personal problem with Kevin on how strange he was acting. They do work together in Showdown: Part 2, when Azmuth suggests that they help him with the helix. Gwen, however, felt somewhat offended when Rook says that she is only attractive by Earth standards. Rook knows when Gwen's birthday is. Rook reveals that they talk a lot in An American Benwolf in London. Gwen even told Rook about Charmcaster, and both of them communicated with each other throughout the episode Charmed, I'm Sure. Rook was worried for Gwen when he does not know about Kevin's whereabouts in The Rooters of All Evil. Rook saved her from being hit by the Rooters in Weapon XI: Part 2 when she was knocked out.
A key similarity between them is that they are both very intelligent. Rook is shown to be polite and formal towards her, even referring to her as "Miss Tennyson."
Michael Morningstar, aka Darkstar, is perhaps Gwen's most hated nemesis. She hates that he is such a misogynistic jerk who takes advantage of women like herself and Charmcaster, and is creeped out by his continued calling of her by "lovely Gwen".
Due to being controlled by the Lucubra Gwen inadvertently became connected to the Diagon's influence. The Diagon saw her as a means to escape his prison, and refers to her in a respectful matter. While under Diagon's mind control, Gwen was his devoted servant and wished (or rather controlled) into attempting to release her master, and was only free once Kevin put metal over her head to stop the Diagon's mind powers. On the other hand, Gwen both fears and hates the Diagon for the trauma of being brainwashed by him. Other than this Gwen and Diagon do not have much interaction.
Even when finding out that Looma was Kevin's fiance, not much animosity was between the two girls. More of her disappointment went out to Kevin who duped Looma in the first place to get what he wanted. Gwen felt more sympathetic towards her, trying to imagine how Looma must've felt being in the situation. When Gwen tried to reason with Looma, she insulted Gwen by calling her a "puny human" and attempting to assault her. Ultimately calling her a mere "girl", Gwen was forced to become serious and finally fight back.
Hex used to be one of Gwen's worst enemies in the first series, bearing a grudge against her for foiling his plans for world domination. But in Omniverse, Hex reforms and becomes Gwen's favorite teacher at college, the two of them bonding over their shared affinity for magic and mutual concern for the well-being of Charmcaster. Gwen defends Hex from Ben's constant accusations of still being evil, though she admits that him giving her a B- is pretty evil of him.
Charmcaster developed a grudge against Gwen in the first series, because Gwen stole her spell book and learned magic easily. In Alien Force, Gwen and Charmcaster developed a rivaliry similar to Ben and Kevin in the first series which is seen throughout the series with Charmcaster trying to kill, hurt, or take from Gwen in any way. Their relationship started out the same in Ultimate Alien, until Where the Magic Happens, when Gwen and Charmcaster teamed up in the Ledgerdomain. The two bonded through their experiences and combining of their powers, with Gwen coming to sympathize with Charmcaster when learning her origins.
In Enemy of My Frenemy, Gwen and her team return to the Ledgerdomain to help Charmcaster. However, Charmcaster's long time trapped in Ledgerdomain had corrupted her, and she temporarily killed Gwen and her teammates to resurrect Spellbinder. When Charmcaster's father refused to come back to life at the expense of others, she became saddened by that and her life became empty and lonely. Gwen let Charmcaster go rather than have her apprehended, revealing she didn't hold a grudge against her for her betrayal afterwards, but instead felt even more sorry for her and worried about how she'll be able to move on after this trauma. This pity was maintained in Couples Retreat, where Gwen is very concerned about Charmcaster's well-being when Charmcaster falls in love with Darkstar, and tries desperately to convince her that Darkstar is merely using her for her power. At the end, the two have a truce and Charmcaster gives Darkstar over to Gwen to beat up.
In Omniverse, Gwen first saw her in Charm School, where she was attending class with her conducted by Hex. Charmcaster isn't there to go after Gwen though, but her uncle Hex. She double crosses Hex and turned him into a totem in order to clamm his staff. Gwen tries to talk some sense into her, telling her that she's not a bad person but is damaged and needs help, which started a magical contest fight between the two sorceresses. Gwen was overpowered by Charmcaster and was tired. They had a long fight around the university until the team of Ben, Gwen, Kevin, Rook and Zed led her to the library. Gwen holding the Hex totem in the library allowed her to become more powerful and she sent Charmcaster and Hex to Ledgerdomain.
In Third Time's a Charm, Charmcaster turns Gwen into a totem and puts her in her bag with Adwaita, Darkstar and Hex. Charmcaster playfully plays with the totem in front of Ben, Rook and Kevin saying that Gwen thinks she is much more powerful but now she fits in her bag. Gwen managed to use Charmcaster's lipstick to draw in her bag. Ben tricked Charmcaster(who has turned into a dragon) and Charmcaster touches the magic in her bag and is turned into a totem. Gwen, Hex, Darkstar, Bezel and Adwaita were free, with Darkstar being arrested but Adwaita escaping. Gwen keeps Charmcaster, who is now inside her own bag, in her college room. Gwen happily says that under these circumstances of Charmcaster being rendered harmless, she and Charmcaster can finally become friends, as she pats the bag with Charmcaster in it fondly. Much like with Kevin, it appears that Gwen will never give up on Charmaster and bringing out her inner goodness.