She is a playable character from the start of the game. You can switch between her and Ninja at any time, and both of them train at the same time (i.e. if Lily levels up, so does Ninja, and vice-versa).
You are meant to switch to Lily on the customisation menu which is very hard to find.
Lily has short navy blue hair, a light tan, and bright blue eyes. She has a flower that looks like a pink lilly on the right side of her head.