I kinda dissapointed with Rio, I mean in the office holiday chapter she decided not to bring the bikini coz her boyfriend wont be there to see it, but now she just wearing it and having fun with her friend(idk it's the coworker or former classmate).
I know that she has social live outside her bf but still it's some sort of promise with your bf, she just can ask bf if he want to come along since they made a promise, bf can just ask Mutou to join them so when Rio having their agenda the bf can chill with Mutou. So yeah a bit dissapointed, let just wait how it turn out.