She is sitting in the library, studying. It's her most common pastime, by hours; spending time with her sister is more enjoyable but Jaromira prefers to socialize more broadly and Katarzyna would only get in the way. This is better on net; the two of them share a room, so it isn't as though her sister's time is entirely hogged by the other girls who call this boarding school "home." Or at least "prison." Katarzyna doesn't mind it, though; the school library is excellent, and basic manners and care for the books has endeared her to the librarian to a sufficient degree to lubricate the interlibrary loans process when there's something she wants to read that they don't have.
"Okay, so you probably wouldn't do anything permanent by accident? Do you wanna have me try dropping-and-stopping stuff for you to mess with?"
"Three, two, one," and then Katarzyna drops it, reaching out for the magic Isabella is about to cast.
"Oh wow, you're a power mimic! Kind of! Can you do that from the beginning if I don't stop it, just from me being nearby?"
"Let's find out--I don't think I'm just a mimic, it felt like I could have canceled your cancel--maybe that's just higher-level mimicry, though, considering--"
"Hm, that might be non-mimicry depending on how it works with other kinds of spells but it's sure hard to test with mine." She reaches to pluck the feather out of the air and drops it again.
"I know! Oh, imagine the synergy possible, if I'm around several magical girls with powers that could combine in interesting ways--oh!"
"Ooooooh! - You might not have it full force, a feather doesn't exert a lot of force."