When first introduced, Kelly appears to be a creature made entirely of aquamarine hair, with her only other characteristics being eyes and legs. Near the end of "Goblin Dogs", however, she is revealed to be a teenage girl with brown skin and long bushy hair that usually covers her whole body and reaches down to her ankles. Underneath her hair, Kelly wears an oversized pink-and-white-striped sweater, black shorts, pink boots with aquamarine tassels, and big round black-rimmed glasses.
In "Starcrushed", "Lava Lake Beach", and "Monster Bash", Kelly wears her long hair tied in a ponytail with a pink hair tie. In "Lava Lake Beach" her long sleeves are folded up, and she wears purple sandals with socks.
In "Stump Day", her winter attire consists of a purple sweater, a white scarf and purple pants and she wears her long hair tied in pigtails with a pink hair tie.
In "Booth Buddies", Kelly wears a pair of white boots and a yellow dress with short sleeves, a white collar, and two buttons.
In "Goblin Dogs", Kelly first appears as part of a long line of customers who want to buy Goblin Dogs, having saved a spot in line for Pony Head, Star Butterfly, and Marco Diaz. Whenever Marco tries to talk to Kelly, she remains silent. While waiting in line, the only sound she makes is the crunching sound of eating apples.
Near the end of the episode, however, Kelly finally speaks up when Star, Marco, and Pony Head question whether or not Goblin Dogs are real. Together with her new friends, Kelly enjoys a Goblin Dog, though she admits she resents what she has to go through every time in order to get one.
The end of the episode also reveals that Kelly has a boyfriend named Tad, a short hairy creature who hangs out in her hair.
In "The Bounce Lounge", Kelly joins Star, Pony Head, and the rest of their old Bounce Crew in attempting to save the Bounce Lounge from being closed—mainly to get her mind off her recent breakup with Tad. Near the end of the episode, she and Tad get back together.
In "Starcrushed", Kelly (once again separated from Tad) helps Star get her mind off her strained friendship with Marco. She is also present when Star confesses her crush on Marco to him, just prior to her return to Mewni.
In "Lava Lake Beach", Kelly and Tad break up for good after a continuously on-again, off-again relationship, and Marco helps the both of them through their breakup. At the end of the episode, Marco and Kelly watch the Underworld's Soulrise together, and Kelly wishes him a happy birthday.
In "Monster Bash", Kelly attends the Mewman/monster party and briefly does the "sword hand dance" with Marco.
In "Stump Day", Kelly attends Star's 15th birthday party on the Mewni holiday of Stump Day, where she reveals she doesn't believe in the Stump. She tries to act as a mediator when Marco and Tom get into an argument, and she also gets into a brief argument with Janna over her possibly developing a crush on Marco. During the argument, she learns (to her annoyance) that Tad is still living in her hair. After the guests reconcile, Kelly dances alongside Marco.
In "Divide" and "Conquer", Kelly takes part in the battle against Meteora Butterfly as one of Marco's "Marc-nificent Seven". During the battle, she finds that Tad is still living in her hair, and an argument over the matter leads to her falling to Meteora.
When she first appears in "Goblin Dogs" wrapped around her hair, Kelly shows a quiet nature even when Marco is trying to communicate with her.
- Theme Song (seasons 2 & 3)
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- When Kelly's body is covered up by her hair, she bears a significant resemblance to the Addams Family character Cousin Itt.
- Kelly breaks up with Tad often, as she states in "Starcrushed" that they're on their third break that week. In "Lava Lake Beach", they break up for good (though Tad refuses to accept it, to Kelly's annoyance).
- "Lava Lake Beach" reveals that Kelly's hair contains a pocket dimension of some sort, and that Tad has been living there.
- Kelly often meets up with Jorby to spar with each other.
- "Divide" shows that Kelly owns many swords.