Ichika is the bride. One, she was talking about kissing as an actress. Two, she used those same "acting" skills to kiss Fuutarou. Three, the bride and fuutarou were talking about how after that kiss, the kisser chanced drastically, and no other quint changes drastically than Ichika. Four, Ichika at the time was conflicted with her feelings for Fuutarou, hence there is a good motive to kissing him disguised.
Four, it can't be Miku and Nino because they would have bragged about it.
So, it could be Yotsuba (likely as well), or Itsuki (unlikely), or Ichika, and Ichika has the best chances.>>188425138
It's likely that she chases him after a while.