There are a lot of reasons to dislike Mashuu. He basically tramples on Ogata's feelings for one. No, he does not need to reciprocate her feelings, but he now indisputably knows her feelings for him and shows almost complete indifference. He gives a passing thought to them, and then he goes on his way, acting like it doesn't affect him, that it isn't all that important. This is the obvious stuff.
But that shows the problem with Mashuu. I would actually point out the scene in the cafe where the two are studying. Satoko says she talked to Mashuu's dad. Then Mashuu says it is his responsibility to talk to his father. Uh, no. It is not Mashuu's responsibility. Yes, Mashuu's perspective would be very helpful in the situation. No, it is not his responsibility. The issue is between the guardian and the outsider, between the father and Satoko. Mashuu's primary and overriding concern is to make sure he gets to spend time with Satoko. He does not care about who feels wronged or who may be hurt. He only cares about being with Satoko. It's a particular kind of selfishness and self-conceit.
I probably sound super critical, but I recognize that he is a child and is not that mature. But then that needs to be emphasized, that he's not actually mature.