Having played Undertale myself, I think I know what you're talking about. I'm not sure I'm the best person to ask, but I can issue some decent suggestions.
You could name the character level DIS (DIStance), because when you kill massive numbers of anything, you start to distance yourself from the events. AP (APathy) could work as well, for similar reasons. Or BL (BloodLust), if you're feeling
particularly dark. KP (Kill Points) or EP (Evil Points) would work just as well for EXP. Or you could just retool the default acronyms, like Toby Fox did (Level of Violence and EXecution Points, instead of LeVel and EXPerience).
That being said,
@Diretooth has an interesting point. I'm not sure exactly how you'd have events keep track of the character's exp
without a system variable, and it's likely easier to
just use the system variable.
Then again, my own game is suffering from massive feature creep,
so what do I know about simplicity?