Gwendolyn "Gwen" Tennyson is a Human/Anodite hybrid Plumber, the paternal cousin of Ben Tennyson, and (along with Kevin Levin) was a front line member of his team during the first 3 shows. Gwen was first introduced when her parents sent her on a summer-long road trip with her Grandpa Max and her cousin Ben. She has, as Verdona refers to it, "The Spark".
In Ben 10: Omniverse, like her boyfriend Kevin, sixteen-year old Gwen is a recurring character rather than a main one, as she went off to an Ivy League college, but visits occasionally.
In the original series, Gwen had short orange hair held by a blue hair clip and green eyes and wore a blue shirt with a cat logo on her chest and white capris. She wears a blue hairclip in her bangs. Her lucky girl outfit is worn in Lucky Girl and Tough Luck.
In the first and second seasons of Alien Force, Gwen wore a white shirt under a dark blue sweater, black stockings under a black mini skirt, and black high heels. Her hair has also grown longer and is now down to her waist. In the third season, she wears a red shirt paired with a black vest, gray pants, and black flats. Her hair is worn in a high ponytail. In her Anodite form, she looks like a mini version of Verdona. At the beach, Gwen wore a black bikini. Later on, she wore a white one. In Ultimate Alien, Gwen's outfit is similar to her outfit from the first two seasons of Alien Force, but with a red sweater while her hair still is now slicked back in a ponytail.
In Inspector 13, Gwen is seen wearing a violet tank with a white skirt and tennis shoes.
In Omniverse, Gwen still has her ponytail, but it's much shorter. She now wears clothes similar to her Alien Force seasons 1-2 outfit, except she now wear long white socks that go up to her knees with black mary janes. She now also has freckles.
Later on, she has shorter hair, which is now orange again, with her blue hairclip, a light blue T-shirt with her cat logo on it, a blue plaid miniskirt and wears blue-tinted glasses. In her new Anodite form, her skin turns purple, but her hair remains the same length, but is now made of mana and is pink and her eyes also turn pink. Her glasses also become a blue mask and she now wears blue gloves and her shirt is a darker blue with a light blue stripe. Her shirt cuts off and reveals her midriff and naval. She also wears blue boots, a dark blue miniskirt with a light blue stripe and a belt. The cat logo on her shirt also becomes black with pink eyes.
11 year old Gwen in Omniverse looks just like she did in the original series except that she has freckles and her cat logo is now bigger.