Nana
Nana starts out slightly suspecious of Shin's orgins when he first joined BLAST, but as time progressed he begin looking out for Shin as an older sister. Although Nana is defiantly not considered as "motherly" (to blast or anyone otherwise), she has an older sister instinc and trys to keep Shin out of trouble. Nana and Shin are rather familiar with each other, and Nana is the first one to express her indifference towards Shin's age when he first joined the band.
From the beginning, I never though that Nana and Shin could be considered as a pairing. It's true that she's interested in him, but that relationship comes across as concern rather than love. She's more of an older sister/family figure to him, so it's unrealistic to consider them as a couple. Still, Nana cares extremely for Shin as well as Hachi (they both lecture him for his piercings and for smoking).
Hachi
Hachi first viewed Shin as an innocent little boy in need of guidence, hence why she was so shocked that Shin was sleeping around with girls for money. At first she mentioned of Shin as "the pretty boy" to her ex.boyfriend Shoujo, and later on she becomes adored with Shin because of his carefree attitude. Shin said that he was "attached" to in chapter18 because "girls are want me for my heart or body, but Hachi's nice to me without wanting anything in return."Shin is extremely dependent upon Hachi and often cries to her when he claimes to be "picked on". In the later chapters of the manga, Shin talks to Hachi about her pregnance, and suggests that she abort the baby. Hachi tells him that she understands his pain of growing up of and not being loved by his parents, and says that she told him this because she doesn't want her own child to grow up thinking of her that way. Shin replies "I wish I was Hachi's child".
At first, I thought that the possiblity of a ShinxHachi pairing was highly possible. Dispite the 5 year age difference, Hachi and Shin has the same childish personality, and I can imagine Shin and Hachi developing a carefully relationship. However, due to the later complication is the manga involving Hachi getting pregnant and engaged to Takumi, I don't see that happening anymore.
Reira
Reira, perhaps, has the most complicated relationship with Shin. Although she is 8 years older than him, she often expressed interested in him. When Shin offered REIRA a price to sleep with him, she accepted it. It's unclear of how Shin truly feels for her, since he often express a rejected looks when REIRA is involved. Later on in the manga, Shin, knowing that REIRA still had feels of Takumi, offered to comfort her by taking her to see Yasu. It's understood that while he had some sort of feelings for her, he couldn't summon it up and express it.
Shin first met REIRA after a Trapnest concert, in which Reira sees Shin and goes "Wow, you're gorgeous. Want to play with onee-chan?" as a joke, in return Shin said "sure, but I'm expensive" and offered her the price of 100,000 yen. Reira confides herself highly to Shin, and they often hang out when Reira is need inspiration. At first she seemed to use Shin as a replacement because of her unrequited love for Takumi, but later on she falls in love with Shin and asks him to "do this not for me, but for yourself". (In other words, don't sleep with her because she's paying him, but do it because he wants to).
I think the possibility of Shin and REIRA pairing is actually pretty high. REIRA first came to Shin for comfort because of her previous love with Takumi, and then she became more attracted to Shin. Also, REIRA looks like Ryokuo, who Shin claimes to be the woman who raised him.