Man if he just introduced himself to his boyfriend's friend like a normal person they woul...

Finally read this series, and I agree. Like, the mentor had every reason to be suspicious and have issues with Shinobu, especially since Hikaru says they don't really communicate unless it's important and he doesn't know if they're dating. Hikaru's going through all this guilt about doing too much when Shinobu isn't really making the same effort, remaining rather private and not being affectionate.