I agree, he's not supposed to be an asshole.
But it's not being an asshole to not help someone who was using you.
Because that's the thing: she was still using him.
He was hurt by the reveal, just that he's so used to it that it's not surprising for him anymore.
So he got all the reasons in the world to hold a grudge.
And even if he was not holding a grudge: why would he want to asociate himself with someone who was using him?
I mean, his whole character until now was "Girls try to get close to me to get close to my brother, not because they actual cares about me, so I won't trust any girl." It's because of that that he was fleeing the heroine at the start.
And the one time we see a girl actually using him... He's just like "No, it's no big deal, i'm used to it".
I'm sorry, but I don't think it's coherent.
I was not talking about his weird tsundere act and desire to trust the heroine. I was talking about the fact that he is extremly wary of her (well,he was way more suspicious in the begining, now he's warming up to her) and he had multiple moments where he was trying to ignore her because of the whole "She will use me too".
That's why i'm saying his reaction this chapter makes no sense:
A girl is trying to get close to him and seems to be interested in him, and only him? "No, it's a lie, I can't believe her".
Another girl reveal that she was trying to get close to him to use him? "Haha it's okay I knew it, now let me help you. Even better: let's be friend!"
I'm sorry but no: she got forgiven way too easily.
Anyone who's complaining about his apathy and how he supposedly acts counter to the initial point of the story, just read this.
You should probably read between the lines more instead of taking what he says at face value. The reason that people were able to use him to approach his brother in the first place was because of his personality. He's too trusting of people, too much of a nice guy, he let's people in really easily. (and also not trusting women isn't the same thing as holding grudges against them, he doesn't actually hold grudges against people cause he's too nice for that)
The thing is, he knows this. He realizes that people take advantage of his friendliness so he now attempts to put up walls to protect himself.
The issue is he's still a really nice person, he can't change his personality; so if someone's got a problem and they ask for help, HE CAN'T bring himself to refuse because that's just how he is.
Those walls he puts up act counter to his very being, so they have flaws, holes. They aren't very hard to break down if you don't strike where he expects you to. She fessed up and apologized, which is something that his "walls" aren't equipped to defend against. Same with houjou, her actions are nothing like the girls that use him, so she's confusing.
Tldr, all that matters is that he's just way too nice to hold a grudge or stay mad. Especially because he's gone through it so many times already. You might think that's dumb, because, yeah, it kind of is. He should be mad; but it doesn't act against his character. It fits it perfectly.