Tomoyo is Sakura's best friend. She is the only who is not related to Clow but knows about the exitence of the Clow Cards and Kero-chan (She accidentally video taped Sakura's first catch of a Clow, since then, she records each one of them). She helps Sakura design the outfits which she wears when hunting for cards. She also records the procedure when Sakura is catching a card and enjoys it a lot!.
Tomoyo is very clear-sighted, she finds out everything hiding before another person does, specially Sakura. (She was the first to suspect about Syaoran's real feelings)