From Tokyopop:

An uncatchable thief is plaguing the city. No one knows her true identity and the police are unable to stop her. She is only known as Saint Tail. At St. Paulia Junior High, a young student named Asuka Jr. makes it his mission to find, stop, and apprehend the elusive Saint Tail. Little does he know that the nocturnal thief is really his friend, Meimi. Aided by a novice nun, Meimi steals to help the needy. Don't miss any of Saint Tail's magical exploits in the tale of this modern day Robin Hood.