Seiichiro is a boy in the third grade of Seiyo Academy. He greatly admires Amu, and in the first episode of the anime, she rescued him from two boys that were assaulting him. He was very grateful, and even asked Amu for her autograph. But she just told him he was blocking her way as well and walked off. His obsession with her appears to have developed since that interaction and now calls her "Amu-senpai". He even began to write an anthology of "Hinamori-senpai's wonderful sayings", shown in Episode 24.
He would do anything for Amu. For example, he trekked down the snow slopes from his grandmother's house when he saw Amu on the television at the snowboarding contest. He also had a fan letter in an earlier episode he didn't get to give to Amu, but was going to.
During Episode 45, he is like the temporary "star" and shows him trying to sing, and practicing. It also shows he is not very good at singing.
Seiichiro is often seen cheering for Amu, admiring her in someway, or proving to Saaya Yamabuki that Amu is a superhero or can save anyone from anything.
Seiichiro is a small boy with light skin and very short, chestnut-brown hair. He have informally grey eyes, and wears large and round glasses. Seiichiro has been seen in his school uniform and a winter's outfit, and also, briefly, normal clothes.
- Seiichiro's full name was revealed in Episode 45 of the anime.
- Seiichiro has earned the abillity to see Guardian Characters since he had his egg extracted in episode 2
- Seichiro may have a crush on Marimo Hatanaka in Episode 45 of the anime as both believed in each other when both had faults.
- In one episode, Seiichiro have a dog named "Cool". Cool is found by Amu during the end of the episode.
- Cool can see Guardian Characters, for example, Su.
- In the anime television series, Seiichiro is voiced by Omi Minami.