Been a long time since Shizuku got her turn at the usual attempted seduction. Umimi has been hogging Riku lately.
Takt wants to have matching swimsuits with Riku and has weak, almost nonexistent reactions to Shizuku.
Shizuku, way to send mixed signals with that "wipe it from your memory right now!" when, IIRC she's already got into way hotter situations before with him. Physically speaking Umimi is leading the race even if Shizuku had the most time spent with Riku.
Hell, his fingers have already been to places with Umimi
But this is par for the course in this manga, which is by now a master class in stalling for time and stretching the plot thinner than the sheer fabric these girls would love to wear for Riku.
I'm just gonna repeat my previous thought that Riku is not as sane as we think, and he's probably just as crazy as Shizuku, Takt and Umimi are. If he was actually, truly sane he'd have dropped the 3 crazies to be with Yako long ago.