Kaitou Saint Tail

Meimi Haneoka, 14, is a normal girl during the daytime, but during the night, she assumes the "position" of Saint Tail, a modern-day Robin Hood who steals from thieves and gives items back to their original owners. She is aided by her friend, Seira Mimori, a nun-in-training, and she is chased by her classmate (and soon-to-be love interest), Daiki Asuka (often called "Asuka Jr.").

(Source: ANN)