>>281173199
I think it's just Fuji never bothered naming him, like how Watashi from Jintai doesn't actually have that as her name. Maybe she thought he's intelligent enough that he would've already had a name he uses but she never asked for it.

>>280552519
MAL and Anilist both list "You" and "Kimi" as some of his names. Maybe he thinks he's been named "Kimi" by Fuji if she says it a lot when asking him stuff?