w/e, i bet you're gonna tell me next that somehow darth mom or even hiromi are generous as well. Everyone on this show but Noe are selfish bastards.
The whole Shin arranging 423 to date Hiromi was him trying to find out if she really had any feeling toward 423. If not, he's hoping this would provoke hiromi into taking the next step up in their relationship. I know, cause I'm a little worthless bitch who did this exact same stunt on a girl before.
Then of course, being a emo bitch she is, Hiromi then goes "if you're fucking with me just because you're a wuss who can't step up, I'm gonna fuck you too by actually going out with 423". You can tell she figured Shin out during that scene in her room, where the 2nd time around her thoughts were played out.
Of course once he realized his scheme has back fired, he goes "AW FUCK!". And gets depressed. That's when he saw realized that since he's getting along pretty well with Noe, plus the fact he had 423's approval. So he asked her if she's into him the first chance he gets. And you know the rest.