Papyrus, Gaster, the kid (c) Undertale: neutral routes get real depressing
He'd probably be utterly shocked (and perhaps a touch humiliated) by Papyrus being able to save a human, which in his mind have always been evil, remorseless, and determined to a fault, with simple kindness - after all, in Handplates he expressed to Sans that he might one day have to kill to protect his brother, and to Papyrus that his eternal philosophy of MERCY was going to get him killed. The human proving their capability for wanton and relentless violence but then also giving up their goal and conceding their determination due to Papyrus's simple offer of mercy would firmly disprove that belief and probably leave his philosophy quite shaken for a long while.
He ultimately would be unsurprised of the human's actions in a Violent Neutral run however - Perhaps he'd internally justify the human only acting out of self-defense to himself as a matter of the human not being developed enough to develop the lust of EXP and murder he'd seen other humans display given they are just a kid after all, or come to believe it to be a result of the influence of Monster figures on them impressing that MERCY has some value (cough, cough, Goatmom). He'd try to fight them and take their SOUL through one means or another, of course, but he would likely go into it having made preparations in case he didn't come back victorious or at all. He'd take the chance given that this does seem to be just a child, but he knows that there is still a chance he'd meet his end at their hands given the species he'd be dealing with.