Towelina is a minor character in The Amazing World of Gumball. She is the girlfriend of Matt, and is first mentioned only in "The Teapot" (but only shown in a photo). According to Matt, she's from Spain. In "The Shell," she appears briefly in person, in the audience watching Gumball and Penny kiss.
Towelina (like Matt) is what her name suggests: a towel. Aside from being blue, she is almost identical to Matt by shape and size. Her distinguishing features are larger, rounder eyes, no headband, visible eyelashes, a green ribbon and green wristbands to go along.
In "The Teapot" when Matt shows Gumball their picture, she seems to be happy and cheerful like him. It could be possible that they have identical personalities.