She is a hilarious hippopotamus, as hinted out by her giggling and snorting, and by how quickly she introduces lots of people (and fellow animals). Like Pudge, she is supportive of Danny.
Tillie is a large hippopotamus. Normally, she wears a blue bow, a yellow-and-blue dress, and a pair of white bloomers. On the other hand, whether she is wearing her sailor outfit or her premiere dress, she doesn't appear to be wearing bloomers.
Role in the film
Both her speaking and singing voices were done by Kathy Najimy, who is best known for the voice of Peggy Hill in the animated sitcom King of the Hill.