There, Undyne calls the protagonist, asking to deliver a letter for her. Backtrack out of the CORE towards the MTT Resort, to the bridge connecting the two places.In Mettaton EX's fight, get the ratings to 10,000 (12,000 if Mettaton still has legs) to end the battle peacefully (this includes using Alphys's phone upgrade to shoot at his heart during some of his combat rounds). Otherwise, go through Hotland and the CORE.If their inventory is full, she waits to give it to them outside Papyrus's house. If Photoshop Flowey and Mettaton EX have previously been defeated, she immediately gives the protagonist her letter.Talk to him to enter the house, then complete the 'cooking lesson' and faux battle with Undyne. Return to Waterfall and go to Undyne's House.Papyrus is in front of her house and tells the protagonist that she got heat stroke, and they cannot hang out with her. Leaving the room while she is still passed out aborts the True Pacifist Route, as she disappears from her spot when coming back.After Undyne passes out from pursuing the protagonist, get a cup of water from the nearby water cooler and pour it on her.After fighting Papyrus, visit his house and complete the date/hangout with him.If one accidentally kills a monster, they can simply reload their save file. FIGHTing monsters, killing the Dummy in the Ruins, or attacking Mad Mew Mew when she can be spared do not cause this route to be aborted, as they do not accrue EXP.

Throughout the game, never kill any enemies or gain any EXP/LV essentially, only use non-lethal battle exits (spare, flee, or battle-ending ACTs).

It is possible to combine this with the following requirements (steps 2-6) and simply backtrack after defeating Asgore and reloading the SAVE file.

Defeat Asgore and Photoshop Flowey, completing a Neutral Route.

Note that killing Asgore or Flowey does not acquire any EXP, as the protagonist cannot SAVE after fighting them. The protagonist can then reload their SAVE file to fulfill any missing requirements as long as they have not gained any EXP/ LOVE. If Flowey was killed, the game immediately returns to the title screen.

He provides a hint on how to earn the True Pacifist Ending (but not if they aborted the Genocide Route). If the protagonist spares Flowey, he appears after the Neutral Route phone call.

Asriel undertale