If hiroshi was like that to his childhood friend then why has he been so nice and caring to the main character? even from the beginning.
Is it just because he knows that she's a prostitute that triggers his protective instincts?
Because this flashback already happened but from Hiroshi's POV back in chapters 40-41. Basically:
-He kept behaving like a dick.
-Nana told him he couldn't be with him anymore and started ignoring him.
-Hiroshi tried to impress her but without actually considering her feelings.
-She got fed up with his BS and told him to fuck off.
-He then actually realized about how he fucked up their friendship/relationship and started to cry feeling regret about how he treated her and her other exes.
-He apologized to every single girl he mistreated in his mind and promised himself that the next time he likes a girl he will treat right and be a good boyfriend.