Endings

Undertale has multiple playstyles available to the player, each with their own ending.

Pacifist
This playstyle requires the player to kill no enemies throughout the game. The player cannot gain any EXP, so will not gain LV, ATK, DEF or HP, forcing them to stay at LV 1.

Neutral
A neutral playstyle is very broad, and involves killing some but not all monsters in the game. The player will have increased stats (most importantly HP) from gaining EXP and LV, helping them survive battles more easily than a pacifist run.

The player is unlikely to kill every monster in the Underground if they do not seek out a large amount of random encounters.

Genocide
A genocide run is tedious, requiring the player to kill all enemies in each area.