Moonlight Diana

What happens when Miku Fujisaki attempts to read a love letter from a classmate's desk? Answer: She gets her picture taken by top student Kei Kurumizawa. Miku really isn't an evil person. The day before she saved a cute little dog from getting hit by a car. The doggy even sported a nice cool bracelet, to match the fur..?

Kei, being the top student of course, blackmails Miku into stealing a sphere from school. If only her friend and crush, Tsukasa Kiryu, could help her out. Oh wait, wasn't that Tsukasa's letter Miku was attempting to read..? Unfortunately, Miku has no choice but to go along with the theft, or suffer the consequences! Thus Kaitou Diana is born ... a thief with a promising start.

(Source: Ganbaru Fan Works)