I really like how Yuu went from teasing Touko by calling her by her first name like it's nothing to not be able to do it anymore. Of course it's sad because she's locking her feelings, but at the same time it shows how much she's changed. It's brilliant.
I really like this manga. It's not the typical "cute girls doing cute things" yuri, nor the typical cliche drama. It has a story deeper than that, and I can't wait to see what's next.
Anyway, it's clear the new guy is someone who was close to Touko's sister, maybe a friend or her boyfriend. Like mentioned above, I don't think he'll cause stupid drama, this isn't that type of manga. Maybe he'll end up helping Touko somehow