"Then I sincerely hope that you caught the humans who wronged you and brought them to the justice they so richly deserved. However- Frisk, I think, had nothing to do with either of those, Doctor Undyne. Perhaps they hid behind Papyrus because they are a child who, prior to coming here, lacked much in the way of effective self-defense options, and Papyrus was willing to defend them? Because, again, child."

Kuja seemed to realize that he'd lost his composure, and he calmed himself, tapping his chin for a moment. "Well. I'll say this, then: kindly don't hound Frisk while you're both here, if you please. Whatever individual humans have done in the past- or even humanity on your world in general- Frisk is not responsible for them. They are, at their core, even to a fault, a fundamentally good person. However, and this is key-"

He was smiling now, as the hand lowered. "I, madam, am not a good person. I'm petty, I'm vindictive, I'm vicious, and sadistic, and capable of the sort of widespread cruelty that you no doubt think every human is capable of, a hundred times over, and it is thanks to a monumental effort and no small amount of assistance from your 'mortal enemy' that I am slowly but surely negating my nefarious nature. So cheer up. With effort and time, you may well graduate from 'aspiring child murderer' to 'mildly unpleasant person'. And, if you don't, and you do give in to that little voice that seems so insistent about killing Frisk, you may rest easy knowing that I will endeavor to save them- and, by extension, you. And if I fail in that task, if I cannot save them-" The smile freezes, and a hand is gently laid upon Undyne's shoulder as he walks past, moving to leave, now, his voice dropping.

"You can be certain I will avenge them. Enjoy the party, Undyne."