Lily is a fairy that heralds the coming of spring. However, this is unknown to both the player and the characters when she appears unexpectedly in the middle of in stage 4 of Perfect Cherry Blossom to shower them with danmaku, bothering neither with words of dialogue nor even a spell card.
During Phantasmagoria of Flower View Lily reappears, many times in fact, to announce spring. It seems her idea of announcing spring is via a dense cloud of danmaku thrown at you at exactly the worst moment, but at least she's trying to do her job the best she can think of. Despite being one of the most commonly seen enemies (due to appearing randomly on every stage) she's the only non-playable character.
In Strange and Bright Nature Deity, she is credited with the power to bring spring to an area the instant she passes by.