I can tell this is an unpopular opinion, but while Chie x Ai is my favorite couple in this series, Shizuka is my favorite character. She's complicated, and at times not a good person, but that's what makes her so interesting. In my opinion, a character isn't truly human until they do something that makes you hate them.
Despite how hard she's trying to be a good person and a good partner for Mio, she's still struggling to get over someone she loved so dearly in the past. This isn't easy for everyone, and it's clearly taking a huge emotional toll on her and, possibly soon, Mio as well. As much as I want to say she doesn't deserve Mio and that they should break up, I also want to see her talk to Mio about her problems so they can figure out how to work through them together. Given how much they love each other, I feel like they'd both be up to the challenge.
She's really well grounded! It's easy for her to remember the good times with Kaoru, who still clearly likes her. That's always going to be tempting when it feels like a safe fallback. And it's less scary than uncharted territory with Mio, which has already had a rocky start.
That's some hard shit to figure out in high school. Especially when she's gotten herself in a situation where there's no one to talk to about it that isn't one the involved parties.
