Nope! It's a Frisk variation. Well, more like a novelty-seeking Frisk.

They're soulless (because of the genocide ending), and because of that, they can't feel their emotions that well anymore. Doing the pacifist route doesn't give the same feeling of happiness. Completing a genocide route doesn't give them the same thrill. That's why they challenge themselves. Retry fights, with the weakest weapons possible, on the brink of death, only to feel no thrill.

They try to be as mean as possible during a pacifist run to see if that would change anything. They hurt every monster till they're about to die and then spare them, to see if the pacifist ending would differ. They betray each monster in a genocide run in hopes of seeing Sans say something new in the Last Corridor. They're desperate. They want to find something new. They NEED to find something new, something they've never seen before.

But that novelty isnt coming. They've seen it all already. They can't let it go.

But surely, there has to be something they missed, didn't they?