In the episode Everybody Talks About the Weather, Kevin said she gets her powers and abilities from her alien bloodline, but she refused to believe him. Little did she know that she would realize he was right in What Are Little Girls Made Of?.
Her powers and abilities are revealed to be of alien descent; her paternal grandmother, Verdona, is an energy being known as an Anodite; from the distant planet Anodyne, and surprisingly she seems to be the only human descendant who inherited Verdona's powers. Verdona tells Gwen that what she and others originally thought of as "magic" was actually the control of mana, a substance of magic-making Anodites; a magic-wielding race. Verdona offers to take Gwen to her home planet and train her in the immense mystical and magical ways of the Anodites, but Gwen prefers to remain on Earth with Ben and Kevin. She dislikes being called an "energy being", referring to her Anodite heritage as revealed in the episode Paradox.
In Unearthed, an alien creature was able to see her as an Anodite, in which she looked exactly like her paternal grandmother's true form, but slightly shorter. In the two-part second season finale War of the Worlds when Kevin was threatened by a Highbreed Commander, she released her dormant innermost Anodite self for the first time, and easily defeated the commander with an exceedingly strong powerful burst of pure magenta-pink glowing energy. She was almost overwhelmed by her own power and wanted to defeat the Highbreed all on her own before Kevin snapped her out of it by saying "I can't lose you". They go out for a date after the second season finale.
During the course of the series she has developed a romantic relationship with Kevin. She stood out for Kevin when Ben questioned Kevin's loyalty. They have also been shown to have arguments, usually starting with Gwen always trying to force Kevin to "ask her out" and him always answering "Don't push me" (which she once did). They even almost kiss in the episode Darkstar Rising.
In the episode Save the Last Dance, Gwen has a dance in her prep school and expects Kevin to ask her out. The episode ended with Gwen and Kevin dancing together in the moonlight. In the same episode, it was revealed that she liked yellow roses.
In the episode Fool's Gold, Kevin appeared to be avoiding her because he was ashamed of his recent mutation due to accidentally absorbing a massive feedback pulse as a result of trying to hack the Omnitrix. She was unaffected by the blast because she put an energy shield around herself. Kevin gave her a locket of the two of them when he was in his human form, asking her to remember him as he was. Gwen said she didn't care what he looks like, and he replied "Maybe not. But I do."
In the two-part third season premiere, Vengeance of Vilgax, Ben and Gwen seemed to have revived their infamous rivalry from the original series, in part due to Ben's cockiness and inflated ego. In the same episode, Gwen poured a smoothie over Ben's head, and he nearly returned the favor. Despite their rivalry, they both still have a strong bond, visible in the episode In Charm's Way, when Ben defended Gwen and informed Kevin that she had been spending all of her spare time trying to find a cure for Kevin's condition.
In The Final Battle: Part 2, Kevin returns to normal after the Omnitrix self-destructs. Gwen then kisses Kevin on the lips thereby reviving their relationship.