It was revealed quite a while ago that she was the girl that fuutarou met as a kid, and has been carrying a torch for him ever since. She actually immediately recognized him since day 1, but kept it a secret.
Yotsuba has a lot of problem with her self worth after their mother died and flunking from their last school and causing her sister to join her. The helpful genki girl act is basically how she copes because she’s ashamed that she couldn’ t fufill her promise to fuutaro to try their best unlike how he completely turned himself around. So she resolved herself to hiding her feelings and to try and help her sisters instead, and has been trying to help them and mostly fuutarou from the background.
tldr, yotsuba is the emotional punching bag of the series.
What fuutarou thinks is still the cliffhanger