i like to think that rui gives tsukushi occasional piggybacks whether its because tsukushi asks her to or because at a certain point rui felt bad about bullying her short height
Listen, this may sound adorable, but, in my eyes at least, Rui wouldn't be able to lift Tsukushi (she's a string bean in my eyes). If she tried to give someone, let alone Tsukushi a piggyback ride, her back would probably run the risk of giving out.
I've already written a fanfiction about this, because I think Rui being a down-bad disaster on the down-low is a golden concept, but Tsukushi is practically a manlet (/gn), thanks to drumming practice. So I'd imagine that she'd be able to lift Rui, and feel more confident as a result, with Rui not minding one bit, because holy arm muscles.