So... yeah like other says, it was the girls that brought attention to her hanging out with Yukiya (and guys) that made him feel that he didn't want to destroy her relationships with her girl friends and then the guys in the group having a crush on her that made him show distrusts for guys in general (also seeing other guys crushing on her as she's popular enforced that) and had him refuse any friendships and led him to being isolated. It's so disheartening to see seeing at how assertive and confident he was in the past with his friends before the term romance came into this equation :/ (Also seems that majority of her elementary/middle school friend group were in Class 6)
It seems Fuji does actively want to be his friend (or at least I hope, it seems the signs are there that he isn't interested in Miku and actually looking out for Yukiya is there), so I hope Yukiya can let his barriers down a little and allow him in
By the way, the most important thing, Nacchan constantly proving as best girl. B) No one can match Nacchan's powers.
Makes me wonder how she met Miku and how they got close to the point of being besties. She wasn't in the elementary school friend group (Or was she and I can't recognize her?)