User blog comment:LakuitaBro01.2/Lak's Video Game Rap Battles: Player Select!/@comment-25263388-20160223024007/@comment-5264252-20160223031948

There are two in-game events that heavily require Papyrus, one being the date with him and the other being him getting you to befriend Undyne. Considering both of those are events that are needed to complete the pacifist run aside from sparing everyone, I'd say that makes him preeeeeeetty prominent. Also, Sans appears everywhere throughout the pacifist run (and neutral run if you spare Papyrus), so I'd say he's still prominent even outside of his boss fight.