Lady M. o Sagase!

From Forbidden Garden:
Chaos and mayhem is afflicting Yoshida Akari's school. There is a ruthless bloomer thief on the loose-and no one is safe!
But on a more serious note, Yoshida Akari's best friend, Hayakawa Maiko has been missing for 3 days now, and it seems as though no one is making any head in the investigation. So, Akari decides to take things in her own hands and questions the last person who was with her best friend, Student Council President Morishita Natsu. He quickly gives her the brush off stating that he is too busy with the bloomer incident.
Who is this mystery thief and just what exactly has happened to Maiko?