Hm, I always skipped the scene when the Akari was with that Tenjin guy but I'm under the impression that Akari love that Tenjin guy, so the question is, why is acting like the MC betray her?
@Erogami
Well, they are technically "married", so they shouldn't do stuff with other people. Also it's blatantly obvious she's been developing feelings for the MC for a while now.
The whole premise behind them trying to get number one on the rankings is to switch partners, so I don't see why she should feel bad about him making progress with the girl he wants to be with.
At the same time his ass shouldn't get jealous when she spends more time with other people instead of him.