Mitsuki Hoyama is the secondary antagonist of the manga series Doubt. She was thought to be the Wolf in the "Rabbit Doubt" game until she was revealed to be a pawn for Rei Hazama, a former child hypnotist. Her father died in the hospital, but Rei posed as her father causing Mitsuki to believe he was telling her to avenge him by "killing all liars".
Before she was placed in her hypnotic state during the events of the series, she and Yu grew closer to each other as time progressed, evident when Yu showed distress and disbelief when Mitsuki came out as the Wolf. Even after she went unconscious, he still tried to save her from Rei, which worked to a certain extent; while he kept her from being killed, she was still under Rei's hypnosis. Rei used it to wake Mitsuki up in the hospital and pull a knife on Yu.
It is unknown what became of Yu and Mitsuki, but Rei is presumably continuing the "Rabbit Doubt" games without getting caught.