Saitou's mostly known for being the mysterious girl behind the fan-fiction that Wada actually reads—stories starring Duke Abel that apparently aren't terrible. She and Wada share a class, which makes things awkward considering Wada's. well, not a fan.
The "Gorilla" nickname came from Wada, who thinks Saitou overdoes the makeup at school. Combined with the whole stealing-toilet-paper-from-the-bathroom thing, it's not exactly a compliment. Wada's pretty harsh about her, though Saitou doesn't seem to care much—she keeps her distance, sitting quietly at the same food court where Wada and Yamamoto hang out, mostly keeping to herself.
You don't actually see her face for most of the series, which works in her favor. She does speak up here and there, but she's deliberately kept in the background visually, making her more of a presence than a character you can fully get a read on.
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