Gwen Tennyson (Classic)/Alien Force

At this point, Gwen had become a black belt in karate but tended to favor using her powers. She was capable of creating mana constructs for nearly any purpose, usually creating platforms to levitate herself (and others), beams to grab enemies, circular blasts to knock enemies back, projecting beams from her eyes that seem to simulate heat vision and opening up interdimensional portals.
During the course of the series, she developed a romantic relationship with Kevin. She stood up for Kevin when Ben questioned Kevin's loyalty. They even almost kissed in the episode Darkstar Rising.
In the episode Everybody Talks About the Weather, Kevin said she got her powers and abilities from her alien bloodline, but she refused to believe him. Little did she know that she would find out he was right in What Are Little Girls Made Of?.
Her powers and abilities were 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 told 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 offered to take Gwen to her home planet and train her in the immense mystical and magical ways of the Anodites, but Gwen preferred to remain on Earth with her friends and family. She disliked being called an "energy being", referring to her Anodite heritage.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 episode Save the Last Dance, Gwen had a dance at her prep school and asked Kevin to take her. The episode ended with Gwen and Kevin dancing together in the moonlight. In the same episode, it was revealed by Ben that she likes yellow roses.
In the two-part first season finale War of the Worlds: Part 1 and Part 2, 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".
In the two-part second 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 had 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 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 Final Battle: Part 2, Kevin returned to normal after the Omnitrix self-destructed. Gwen then kissed Kevin on the lips. She and Max crashed Vilgax's ship in the ocean while Ben and Kevin fought Vilgax.