Endings

Undertale has multiple endings, although there are only three primary endings, there are many slightly differing neutral endings.

This page contains spoilers for the entire game.

Pacifist
This ending requires the player to kill no enemies throughout the game. Additionally, the player must befriend Papyrus, Undyne, and Alphys. The player cannot gain any EXP, causing them to not progress past LV. 1.

Post No Mercy Pacifist

 * After having finished a No Mercy Run where the player has sold their soul, any Pacifist Run which proceeds it will have a modified epilogue where Frisk is shown in bed to be indeed still possessed by the first human. Alternatively, if you choose the second option, when prompted by Toriel in the epilogue, the group photo with Frisk and everyone else will show everyone's head except Frisk's crossed out in red.

Neutral
A neutral ending is doing either of the following: A neutral run ends with the player escaping the Underground alone. There are a variety of different possible endings depending on the actions you have taken throughout the game. Some of these endings are detailed on the Neutral Route page.
 * The player doesn't kill every enemy in every area, bosses included.
 * The player does not befriend at least one of plot pivotal character, such as Undyne, but also does not kill every enemy in every region, bosses included.

No Mercy
A No Mercy run requires the player to kill all enemies in each area (Ruins, Snowdin Forest, Waterfall, Hotland/Core). As well as kill all of the bosses. The player is warned several times by Sans that they will have a 'bad time' if they continue in this direction.

Completing a No Mercy run makes it harder to start a new game, as the game's world is destroyed in the ending, requiring the player to sell their soul, and this also irrevocably alters the pacifist endings.

A "No Mercy" run involves killing any enemy that engages you, but not going out of your way to kill all enemies in each area. However, this is still functionally a neutral run.
 * Outside of this Wikia, some fans will refer to a Genocide run as a No Mercy or Bad Time run, due to moral issues with the word genocide or not wanting to spoil the means to this ending.