FriskXAriel, but not as their current age of course. Although I like this one as a brotp at the same time. xD
UndynexPapyrus is my brotp. A main reason why I like it is because when I first saw them interacting at the Waterfall, I thought that their relationship wasn't great and that Undyne didn't like Papyrus, and I thought that Papyrus was a bit intimidated by her. So I found it very surprising that they turned out to be close buds and the two were really comfortable around each other, and that Undyne cares for Papyrus' safety. I think that also shows a lot about Undyne, that she's not a nasty person that she appears to be if you don't know her.
I also like Naptstablook and Mettaton as close buds.