Takuto is a shinigami, AKA a Death God, partnered with another shinigami named Meroko. They are assigned to take Mitsuki's soul. His shinigami costume is a cat outfit with a backpack with wings, without which he would not enjoy the power of flight because he is not yet a full shinigami and hence has not sprouted wings of his own. He is capable of transforming into an anthropomorphic cat plushie. Takuto transforms Mitsuki into a 16-year-old to help her participate in a singing audition.
As the story continues he remembers his past life again for Mitsuki, but he was not allowed to. That causes problems... And even his love for Mitsuki is influenced by it.
He was a singer in Mitsuki's father's band, and saw Mitsuki a few times when she was small. Also, the name Mitsuki was decided by Takuto. This is shown in the manga.
When he was around 16yo, he had the same throat tumor that Mitsuki has, and he had the surgery and lost his voice. Because of this, he ends up killing himself. In the manga he jumps from the ceiling of the hospital, in the anime he drives a motorcycle down from a cliff. He couldn't imagine living without being able to sing. However, he didn't die, but went into a coma (at least in the manga), reason why he's not a good Shinigami.
