FUN FACT ~ Level 17 is not actually the highest level that you can be in a Neutral run.

It's Level 19.

Now I know what you're thinking. "Bullcrap! You fight Sans at Level 19!" And that's true... but it's possible to not fight him at Level 19. I tested this recently & this is what I discovered...

Here's how it's done... (I will go into detail, so bear with me.) It's a Genocide run all the way through with an exception. Kill everything in the Ruins + Toriel, kill everything in Snowdin + Papyrus, kill everything in Waterfall + Undyne, (TRIGGER METTATON NEO <-- This is important!) kill everything in Hotland/Core + Mettaton NEO, BUT WAIT!!. There is ONE monster that you Spare between Hotland & the Core...

Muffet

The reason why Muffet is so important to this equation is because of how Mettaton NEO reacts to the whole situation. You know how Mettaton accuses you of holding back if you don't kill all 40 monsters? Well, he only cares about those 40 monsters. Mettaton doesn't give a frick about Muffet. He doesn't accuse you of holding back if you Spare Muffet. His coding only responds to the 40 monsters & NOT Muffet. So even if you Spare Muffet; As long as you kill all 40 monsters, Mettaton will still say "Guess you don't wanna join my fanclub?"

This means that Mettaton will still bump you to Level 19, because he assumed that you killed Muffet. Sparing Muffet aborts Genocide, which means that when you get to Asgore's Castle, the monsters will still tell you the story. You will fight Asgore at Level 19 & after defeating Omega Flowey, you will get the "Alphys Ending"

A few side notes... 1.) After Sparing Muffet, Hotland & the Core will revert to their normal music. 2.) Bratty, Catty & the rest of MTT Resort will still be abandoned & Burgerpants will still have his Genocide dialogue. 3.) Mettaton NEO's health bar will drain instantly, opposed to slowly despite not being a Genocide run anymore.

I found this to be fascinating, so I just thought I'd share this. If you don't believe me, try it out for yourself.

Actaully, you can hit 20 in a neutral run. Each time you pick on Loox, you get +5 EXP when you finally kill him. Toby didn't cap this either, so you can hypothetically get 99999 EXP from killing one Loox, the EXP required for level 20. It would just take an incredibly long time to pull this off.

All found on reddit

https://www.reddit.com/r/Undertale/comments/5mgfgu/fun_fact_level_17_is_not_actually_the_highest/