Yuu realizes she loves Maki. She doesn't try to get him to leave Umi, and she does feel guilty about loving her best friend's boyfriend, but she still ends up making her feelings clear.
As you can imagine, this causes some serious issues for the three of them as a friend group. Maki absolutely has no interest in anyone but Umi, but it's hard for Umi not to feel any insecurity. She also doesn't really hate Yuu for feeling how she does, but it's all quite a mess.