Nino and Fuutarou have the most developed relationship so far from what i can tell. I mean as characters.
In the future chapter of the wedding, Fuutarou was still wearing the charm Raiha made for him, but between which characters has that served as plot device? Nino and Fuutarou/Kintarou ofc, it went from Kintarou to Nino, from Nino to Fuutarou when he was sick, later Nino took it back and then gave it back to Kintarou.
Nino and Fuutarou have both seen each other coming out of the shower, when he came back to get his wallet, he saw her, and when he was at Nino's hotel room, she made him take a bath, and he came out in front of her only wearing a towel and even said that she shouldn't mind as they had previous experience.
Nino has some strange personality what with the drugging thing, but even Fuutarou was taken aback when in her hotel room he noticed they could talk quite freely and easily. In addition to that despite Fuutarou early realizing Nino is going to give him trouble, he never once was harsh or mean or snapped at her, because most likely he realized her intentions and his position as an outsider made what she did justified. Add to that that he also noticed that what she does is because she loves her sisters, if we remember when Miku asked Fuutarou what type of girl he likes, he said smart - hah, Miku is the smartest going by grades, but good at cooking or housework, that's Nino and also must lover her brother very much, which again is Nino as she loves her sisters.
You might say that each of the five loves the others, but if we look at it, Miku is pretty impartial as her philosophy up till now has been equal for all five, Ichika is focusing on her career, Yotsuba is helpful towards everyone, especially people outside her family, Itsuki is focusing on studies, Nino is the one handling meals for everyone and was shown to even carry around band aids for when Yotsuba would get hurt.
And another thing that was interesting, when Fuutarou was alone with Ichika he was thinking of "if i were to chose" in relation to the bonefire dance, he asked Ichika if they should forget about the whole thing, implying he'd rather go with Nino? This one is strange, because it was Miku that basically set up Ichika with Fuutarou for the dance, while Fuutarou as Kintarou got roped into Nino proposing for the dance...
and then there's the whole thing of Rena saying goodbye to Fuutarou, Nino saying goodbye to Kintarou and now we come full circle with Fuutarou saying goodbye to Rena just as the arc ends...