Seiji was first seen in the manga Gokinjo Monogatari (Neighborhood Story). He applied as Mikako's mother's assistant. His dream was to study abroad as a hair stylist, but he used to go to Yaza-gaku. As he spends more time as Mikako's mother's assistant, Mikako grows close to him and starts to view him as a brother (though at first Tsutomu isn't very approving of her and his relationship). He appears in Paradise Kiss a little, rumored to be George's lover.

There is a running joke with him being an alien in Gokinjo Monogatari and Paradise Kiss. In Gokinjo Monogatari, Mikako strangely gets the idea that he can transport, which in her mind of course only leads to him being an alien. When she asks him if he is one, he jokingly says he is. You can tell Mikako even told her little sister Miwako that he is an alien, because Miwako says that he is from "Glitter Planet" (or "Planet of Sparkles" in the dub) in Paradise Kiss.

(Source: Wikipedia and others)