Yuji Kayama was an anime-only character who showed up during the Stars season. He captained the football team at Juuban Municipal High School and had what looked like a solid athletic future ahead of him.
One day after practice, Chuuko Nezu approached him wanting an interview. He blew her off, saying he didn't have time. That's when she revealed herself as Sailor Iron Mouse and yanked out his Star Seed. It turned out to be a blank, and the whole thing triggered his transformation into the Phage Sailor Guts.
Usagi happened to be in the area when it went down, so she transformed into Eternal Sailor Moon to fight him. She wrapped up the battle pretty quickly with Starlight Honeymoon Therapy Kiss, turning him back to normal.
Content compiled by AnimeList.moe from publicly available sources.
