Look, it's a simple fact that whoever kissed Fuutarou under the bell is the bride.
She is not the kisser because of her inability to run.
She is not the kisser for a number of reasons. First, we know that Miku had not made any move on Fuutarou after her livingroom confession because of feelings on inadequacy and because she told Nino such. Additionally, Fuutarou was able to see through Miku's Itsuki disguise by looking at her body language, specifically by the way she walked from behind, which he clearly saw the kisser walk away from him.
Another quint unable to maintain her Itsuki disguise. However, she is still a possibility.
Another quint that cannot be fully counted out, but she has not displayed any strong romantic feelings towards Fuutarou and she also was easily flustered by just holding his arm for a photo. Additionally, why would she betray Yotsuba who she knew was pining after Fuutarou? It doesn't add up.
The strongest case for the kisser. Not only did she declare her intent to kiss him before the trip ended, but we can see several mannerisms of the kisser which indicate Nino such as the kisser's characteristic determined stride which was foreshadowed earlier in the trip and other times. There is also the indication of the bride's specific tastes for the wedding which match up with Nino. There's also the fact that Nino's part time job boss is sitting on the bride's side while none of the other quints' acquaintances (Ichika's managa, Rena's student) are present. Another piece of foreshadowing is Nino's comments about getting her ears pierced before marriage. It also makes little sense for Nino to give up on trying to kiss Fuutarou, given her personality and relentlessness.
In all likelihood, Nino is the kisser, the bride and Fuutarou's love interest. The line of reasoning also fits in perfectly with Fuutarou's comments regarding love and his initiative to buy a dating guide. Elementary, my dear anonymous.