What is wrong with/confusing about/makes Kirishima an interesting character could fill books.
- He grew up rich with absentee parents and ended up getting raised by his spinster great aunt.
- He was apparently a very good student and judo champ who was bullied, along with a female classmate in middle school. Its not clearly what exactly happened as yet, but the scuttlebutt is that he beat 20 boys his age and older so badly that all broke at least one bone.
- One could infer that based on how he acts when Yoshino is in trouble, that Kirishima failed to protect a female classmate who was also being bullied and still feels guilty about it, which is why he goes nuke when his woman is threatened.
- His aunt sent him off to live with her brother Gaku and it's not clear what Gaku did to the kid, but according to several extras drawn by Konishi, Kirishima lost his virginity at 12 and had '4 to 5' older girls 'on the side' when he was in middle school.
Fandom speculation is that the bullying and absentee parents, along with the possible influence of living in what is a hyper-masculine environment (the yakuza), might have triggered a trauma response in the form of hypersexuality. Or maybe he's just a healthy teen boy. Either way, it seems as though Kirishima uses sex to connect with someone he can use as part of his network, but does not actually enjoy it.
This might also explain why he has no friends and has difficulty connecting with Yoshino. He mentions at one point possibly having a friend but the association going south, which might be referring to the female classmate who was bullied. His aunt describes him as an honor student and says he used to be competitive in judo— the time he spent studying and training might have contributed to his lack of socialization and awkwardness. It could also be another side effect of being part of a wealthy family with reputed mob ties, or his suffering a traumatic experience which essentially forced a personality reset to be the most inoffensive, pleasant person he could be so as not to incur the wrath of his peers— something he still manages to do by dint of suppressing his true emotions, even around his women.
It's also clear that at least one of his women, a college student 3 years his senior who started seeing him when he was about 15 years old and she was a college freshman, sought him out specifically because of his yakuza ties.
Further, while he has the tattoos— which is the easiest and most identifiable characteristic of a yakuza member, Kirishima has said on more than one occasion that he does not want to be yakuza. Why? He would suck at it. By his own admission he is not able to make friends easily— noting at one point that Yoshino is possibly his first and to date only friend (although his relationship with Tsubaki seems friendly). He is self-aware enough to know that his temperament and general attitude makes him unlikeable and even angers people. Shouma seems to enjoy reminding him of that at every possible turn, stating that 'no one likes/could love you' on several occasions.
That said, Kirishima has built a network of associates and contacts that he uses to benefit the group or himself. He has a basement room, which the Miyama group had previously used for detoxing members, where he has files, a computer, and other gadgets enabling him to keep tabs on people, and records of his connections. It's possible that rather than a yakuza member he's headed toward becoming a 'black curtain operative' or information broker.
The way he goes nuke when Yoshino is threatened doesn't bode well given that there's a plan in motion to kidnap Yoshino to destabilize Renji's position as the heir apparent to the head post of the Kirigaya group.
