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.