Tetsu is a member of AL4, alongside Sugimori Ren and Asaka Narumi. He goes by the moniker "the General."
He, Ren, and Kai go way back—childhood friends since middle school. When Kai first transferred in, both Tetsu and Ren challenged him to a cardfight. Tetsu was the first to step up, though he didn't walk away from that match as the winner.
Things got complicated when Ren developed Psyqualia. During their next cardfight, Ren's power literally threw Tetsu backward, and the shift in Ren's personality became impossible to ignore. Kai couldn't handle what his friend was becoming and eventually left the team, moving back home. He even asked Tetsu to come with him, but Tetsu refused—he wasn't going to ditch Ren, not even then.
That said, Tetsu clearly felt unsettled by what Ren's Psyqualia had made him. Despite his fear, though, he stuck around and stood by Ren's side.
Content compiled by AnimeList.moe from publicly available sources.
