by Forbidden Garden:
Prequel to Sweet Mission
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, Hayaki 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 that 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?

Taken from Michex:

MITSUKI is on top of the world when IMAI, her former tutor and sister's friend, agrees to begin dating her. Even with the seven year age gap, she's determined to make it work, even though she hardly ever gets to see him. But her happiness slowly gives way to doubts when a year passes without ever breaking down the barrier she feels between them. Does Imai-san even like her at all? Or is the person he's really in love with her older sister and he's just going out with her because he didn't have the heart to turn her down? Maybe he doesn't take her seriously because she's still in high school? When she's with him, he always seems more of the serious type---but what is this about his playboy past?!