Seiji first appeared in the manga "Gokinjo Monogatari" (Neighborhood Story). He applied to be an assistant to Mikako's mother. His dream was to study abroad as a hair stylist, but he previously attended Yaza-gaku. As he spends more time working for Mikako's mother, Mikako begins to see him as a brother figure, although Tsutomu initially disapproves of their relationship. Seiji makes a brief appearance in "Paradise Kiss," rumored to be George's lover.
A running joke throughout "Gokinjo Monogatari" and "Paradise Kiss" involves him being an alien. In "Gokinjo Monogatari," Mikako whimsically believes that Seiji can teleport, leading her to think he must be an alien. When she asks him if this is true, he playfully confirms it. It’s evident Mikako shared this belief with her younger sister Miwako, who refers to Seiji as being from the "Glitter Planet" (or "Planet of Sparkles" in the dub) in "Paradise Kiss."
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