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Two things:
- 1.) Too rare to trope, since very few anime have any trans characters.
- 2.) Aoi isn't a trans character, he's repeatedly said to be a crossdresser.
Elaborating further from point 2: In his introductory episode, he said he's fine with being addressed with either male or female pronouns, but he admits to being a guy. Later episodes remind the audience of that fact anytime another guy assumes Aoi is a woman and hits on him. Aoi eventually has to explain that he's crossdressing, or he'll simply show them by unbuttoning his blouse.
One episode even has Yoriko lament the fact that a guy is more feminine than they are, because Aoi gets hit on more than any of the women combined.
There's this entry on You're Under Arrest! and I'm curious if it should be moved somewhere else: