Short answer? Mizuki probably

Long answer: It sorta depends on when exactly? like junior high and early game yeah neither lol, but when they are slightly older? in the scenario where they do fall for each other in the traditional sense and are aware of it, i do think confessing would actually be pretty second nature for them? Not necessarily bc they expect the other to reciprocate their feelings, but bc whatever outcome a confession might bring, it probably won't change their relationship all that much, and I'd imagine they would both be pretty aware of that. Their indifference to how the other perceives them has always been a pretty key aspect of their relationship and i do think it applies to potential romance as well. Ofc, this comes from someone who has a merely theoretical understanding of romance as i do not . care for that personally, so I might be the wrong person to ask about this lol. But anyways, the only reason I'd imagine Mizuki would be the first to confess in this scenario is more bc she'd probably be the first to realize her feelings, like by a mile. In fact, I have a hard time seeing rui realizing this on his own at all with Rui's conception of love being infamously blurry and all lmao