50% sure It’s Ichika, she announces she isn’t planning to lose to Miku. She has the same earring position like in the bride panel as well so she’s going offensive.
30% Possibly it’s Yotsuba, She’s the dark horse and probably Rena. Her lingering feelings and realization that her sisters are starting to move forward might of made her go on the offensive.
10% For Nino. She was already aggressive in her love pursuit. She also just realized Miku and Ichika feel the same way. I’m not sure she would attack right after this remark then again she confessed twice so there’s a chance she’s very bold.
7% For Miku. Miku just planned on getting serious and shows her resolve, it would be out of character that she would just kiss right after this development.
3% Itsuki. She seems to be uncertain and might have a secret. She really hasn’t had much development as a love interest. If I had to think of a reason for the kiss, probably unknown feelings to her making her do the act as gratitude for Fuutarou’s help to her sisters.