To complete the True Pacifist Route, Frisk must have first completed the Neutral Route.

If Frisk has completed the Neutral Route without gaining any EXP or LV (i.e. without killing anyone), it is possible to backtrack in the existing SAVE file to complete any missing friendships, then achieve the True Pacifist ending.

If Frisk acquires any EXP or LV, they have to reset the game to reach the True Pacifist Ending.

Friendships
While Frisk befriends many of the monsters in the Underground.