Alright, I read all the comments and put my thoughts together.
1. I think harem ending is officially out.
None of the girls seem to be into sharing Fuutarou and neither breaking the law. Plus, I doubt their dad would accept it, even after two years.
2. Rena is their mother's name.
Other people already commented this. This would make the most sense. Grandpa also said "After I lost Rena" which makes it more likely to be someone close to him.
3. "Old Rena" from Seven Goodbyes arc could potentially be the kisser.
This is entirely speculative and I don't have any proof. Think about it, the one who disguised as "Old Rena" has to care about Fuutarou enough in order to guide him towards the right path.
If this is true, it opens up many possibilities:
3.1. Old Rena knows that "Past Fuutarou" (when he had his hair dyed blonde) was with one of them.
3.2. The kisser had already cared for Fuutarou even before Miku developed a crush on him, effectively making her "first girl" which holds quite a lot of narrative value.
3.3. Aside from Nino, someone else realized the presence of Past Fuutarou.
Unless... Nino is Old Rena. But I don't think so since she wouldn't go to such lengths for someone who recently broke her heart (of my memory serves correct)
4. Their marriage may be far beyond two years.
Take a look at this picture.
4.1. The photo of the quints being themselves with the Grandpa being happy was not shown in the chapter.
Considering the foreshadowing of his death as well as Fuutarou returning with the quints after being able to recognize them, it is
quite likely for this photo to be taken at their second return.
Further proof that this is taken in the future:
4.1.1. Nino has her butterfly pins in a different location
4.1.2. Itsuki is using glasses
4.1.3. Ichika looks more slim
4.1.4. Yotsuba is using a different ribbon
4.1.5. I can't see Miku's headphones and her eyes look thinner
4.1.6. Grandpa looks more wrinkled
4.2. "This is something I overheard. Apparently, on that day, they'd already-"
THEY'D ALREADY WHAT? GOSH DANG IT, NEGI!!!!!
Well, I'm breaking this into all the possibilities.
4.2.1. "They'd already kiss."
If this is true, then the marriage would, in fact, take place
five, and not two, years in the future. Considering the timing of the storytelling, it seems to imply that the current bride who is with Future Fuutarou is the kisser in this chapter. However, this is Negi we're talking about - it could just be a red herring.
Therefore, the other possibilities is:
4.2.2. "They'd already met before us."
I'm looking at you, Old Rena.
Also, can we just talk about how morbid it would be to dress up as a stranger while using your deceased mother's name?
I was considering that this was about them meeting as kids with Past Fuutarou but the timeline doesn't make sense, which outrules the "Nino is the bride, confirmed!" hypothesis since she was the girl Past Fuutarou met. Plus, this plot line haven't been developed yet:
4.2.2.1. What impact did their old meeting bring to the story, aside from Seven Goodbyes arc?
4.2.2.2. How will the other sisters react if they know that Nino and Fuutarou had met long time ago?
4.2.2.3. Did they make some kind of promise when they separate? This is just Nisekoi all over again but a new plotline is a possibility.
4.2.2.4. Where were the other sisters when Nino met Past Fuutarou?
Also, going back a step,
4.2.3. Who did she "overheard" from and which of the quints is "she"?
New plot line, guys!
Oh, and...
4.3. I think this 4 bit has confirmed that the manga is far from over so calm down guys...
Then again I could be wrong.
5. I'm gonna guess who was the kisser.
5.1. Yotsuba is a wild card.
5.1.1. She shows no romantic interest to Fuutarou, at least yet.
5.1.2. Negi isn't showing us her inner thoughts which just makes her harder to predict.
5.1.3. Her hiding her personality cannot be ruled out since she's such a mystery. For all we know, she could be hunting for Fuutarou from the start but was hiding behind her cheerful, airhead mask.
5.1.4. She was only foreshadowed to like Fuutarou
twice in the entire series. I tried looking for the pages but I gave up so correct me if I'm wrong and trust my words.
Conclusion: Yotsuba can't be predicted.
5.2. I'm ruling Nino and Miku out.
As much as I wished that it was Miku, it simply does not makes sense.
Nino
5.2.1.1. She would want Fuutarou to know that it was her.
5.2.1.2. Considering that she'd barge in the mix bathing to tease Fuutarou, I doubt that she'd run away
5.2.1.3. It's not her personality. I don't think she would try to tip toe to try and kiss Fuutarou then push him to reach his lips.
Miku
5.2.2.1. She'd also want Fuutarou to know that it was her.
5.2.2.2. She's quite patient in taking it step-by-step so I doubt that she would take another action so quickly.
5.3. Itsuki is ruled out as well.
5.3.1. Her relationship with Fuutarou now is "friendly"
5.3.2. She's supporting Miku so I don't think backstabbing is her way.
5.3.3. She's busy eating chicken.
5.4. Ichika is the most likely candidate.
5.4.1. She haven't taken her first step yet
5.4.2. She's conflicted with her emotions. On one part, she want to help Nino as the eldest. On another part, this arc has developed her character to be more self-care so for all we know, this could be her character development.