She seems to be more observant than Kokoro, because when Kokoro mentions to Makoto about her sister (Kotonoha) wanting to dance with him at the festival, Uzuki notices the look of consternation that comes over Sekai's face. She speculates that perhaps Sekai is actually his girlfriend, something Kokoro adamantly denies.
This does not sway Uzuki's opinion, as she simply remarks "an adult situation, how mature". She then announces that boyfriendless-Kokoro would not understand, which sparks a complaint from her friend.
In Cross Days, Uzuki come to Sakakino Highschool as her cram school teacher and boyfriend, Kyouichi Kasannoin is helding the "Rare Book Lovers" event which is a front for his sexual activities. It's unknown if she is aware of his infidelity or not but she most likely is.