Undersea Gal is a female sea-creature citizen of Halloween Town. She has a green and scaly mermaid body, webbed hands, with fins on her head. She has yellow eyes and blue lips. Sometimes she is seen wearing an eye-mask while coming out of the water. She is often seen as a very grotesque creature, helping out the citizens of Halloween Town during their preparations for Christmas, and ultimately finds strange things inside of her lake, such as an unknown severed head.
Undersea Gal is first seen during the musical number "This is Halloween" where she is seen celebrating Halloween with the other citizens. She is later present with the other citizens attending the town meeting when Jack returns to Halloween Town and shows the citizens his discovery about Christmas. Later during Jack Skellington's plan to take over Christmas, she is amongst the other citizens helping Jack take over the holiday and later watches Jack bring presents to the children. However, it turns out that this plan was a bad idea due to Jack being shot down by military units.
At the end of the film, Undersea Gal notices Jack's return to Halloween Town after being revealed to have survived. Later when Santa Claus brings Christmas to Halloween Town due to Jack saving him from Oogie Boogie so that Santa can undo the mess Jack did in the real world, she is seen making a mermaid snow angel.