Ilia is one of Link's best friends in Twilight Princess. She likes Link's horse Epona very much. In the beginning of the game she'll take Epona to the Ordon spring and wash her. When Link comes there, she'll say that she washed Epona for him. She is also getting mad at Link when she sees that Epona is injured, but she'll forgive him later when Link followed her back to Ordon spring. It's there she and Colin gets kidnapped and the quest begins.
I don't have much to add, but I think she has a crush on Link.