Room 707
Because of Shin's refuse to return home, he often crashes over at varios girls' place. We see Shin the most in appartment 707 with Nana and Hachi as he mostly freeloads off of either the two Nanas. In the manga, Shin referrs to Nana as "Mommy" just to annoy her. Still, Shin spends a good amount of time hanging out in Room 707 with Blast. Room 707 can be considered as an important factor in Shin's life; it represents Blast and his exsistance and importance in the band. Without Blast, he would probably still be another kid wandering around the streets of Tokyo as a semi-prostitude and free-lance bassist. Blast lead him to the road of fame and the profession music industry.
Nobou
In the manga, it's mentioned that Shin sometimes stay with Noubu when he has no place to go for the night. Nana even jokes around and say that Shin practice making out with Noubu when they were under the impression that Shin was gay. The fact that Noubu offers Shin with a place to stay prevents him from hanging around "in ladies pads" for fun, and it also gives him more confidence for not having to sell himself to stay somewhere for the night. Also, Noubu offers Shin a sort of support as an older brother, and in mother of the manga Shin stays with Nobou.
Reira
After offering REIRA a price somewhere during the middle of the manga, Shin often returns to REIRA for "business". To put it bluntly, he sometimes sleep around with Reira for the night.
Nana's opinion of Shin's behavior is never mentioned since she often feels treatened by Trapnest. This curiosity is cause by the fact that Nana felt like she (or rather Blast), lost to Trapnest after Hachi and Takumi decided to get married after Hachi found out about her pregnancy. Nana often feels that her brand represents who she is, and she tries to disencourage association with Trapnest, so Nana would be rather annoyed with Shin's "business" with REIRA.