Okay, then... so taking all of that into account, Kyon would probably be one of Ayumu's closer school friends along with Orito, who's basically Ayumu's Taniguchi-type. He'd see Ayumu crossdressing at the start of the series, would probably notice his increasingly weird behavior when it comes to avoiding sunlight, and would eventually meet Eu, Haruna and Sera and find out they're all living in Ayumu's house.

Depending on how interested Kyon is in figuring out the reasons behind all of this, he might end up the one normal person that Ayumu lets in on his secret.