Oh so he does know

I guess how much sympathy he can still have really depends on when he knew. He could have only figured it out during her aborted confession; but it he did know for a while before that then it makes the entire thing really reflect badly on him. The longer he was aware, the worse it reflects on him.

So Otaku-kun really did know Osanai's feelings and yet he acted ignorant about it. He may be scared to watch her reaction if he rejects her or he may be scared to even confront her about it. Maybe he thinks that if he rejected her confession then it will affect their current friendship so he acted ignorant in order to keep the status quo. Im not saying that what he did was correct.

Grahf said:

I guess how much sympathy he can still have really depends on when he knew. He could have only figured it out during her aborted confession; but it he did know for a while before that then it makes the entire thing really reflect badly on him. The longer he was aware, the worse it reflects on him.

At the very least, his interruption here suggests pretty early on he had an inkling, if not outright knew, but for reasons he just didn't take the next step.

Whether the reason was because he didn't feel like risking their comfortable relationship, or he didn't see Paisen in that light (perhaps both), we would need to wait and see.