It’s all conjecture, no evidence. Plus the demo manual explicitly says HP stands for “hit points,“ when that very same demo manual intentionally pussyfooted around what the letters of EXP and LV stood for (because it was a big spoiler).



But here’s some food for thought: only people with SOULS have HP.



Fact: When you do damage to a monster, the HP bar is visible, as is a number representing the amount of damage dealt by your attack.

This is true not just in battles, but even in cutscenes that are out of the player’s control, such as Asgore being murdered in the end-of-NM cutscene:



(And while Sans doesn’t have a visible HP bar (probably because his would be so frigin’ tiny), he does have numbers displaying how much damage was dealt.)



Fact: there’s only one character, in the whole game, who has neither.



Flowey.



In both Neutral and NM, there is no HP bar or damage number when he gets knifed to death. He just…dies. No flashy game interface stuff whatsoever.

He does have a visible HP bar and damage-taken numbers as Omega Flowey:



…but then, he’s got a soul in that fight, doesn’t he? Six, actually.

In conclusion: Flowey most delicate boy in the underground, more fragile than Sans “the Undertale” McSkeleton himself, glass bones and paper skin etc etc.