papyrus is the one who quotes cheesy song lyrics to undyne, gets her all sorts of presents and likes to be by her side whenever he can

undyne is the one who glares at anyone who wants to split them up, aggressively shows her love with bone-crushing hugs and yelling compliments and reassuring statements, and gets the drunk texts of "ILYSM BITCHHHH YOU MY SOULMATE IFLY" at 2 AM

these roles could fit to either person honestly