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Put the letter under the door of Alphys's Lab and complete the "date" with her.If the protagonist discards the letter, they can return to Undyne to receive Undyne's Letter EX, which cannot be discarded.Talk to Undyne outside of Papyrus's house to receive Undyne's Letter.This phone call can also be triggered in several other locations if the most recent save is outside of the CORE and New Home. 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.
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Note that killing Asgore or Flowey does not acquire any EXP. The protagonist can then reload their SAVE file to fulfill any missing requirements as long as they have not gained any EXP/ LOVE.
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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.
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