How to Complete the Pacifist Run in Undertale

Don't attack any creatures. Same as the Ruins, but more difficult due to the more challenging mini-bosses.

Spare Papyrus. You can also Flirt and then Spare until the end of the battle. Just don't kill him. You can fight him until he uses the "Absolutely Normal Attack", which ends the battle faster since he always uses it one turn before death.

• If you're having trouble with this battle, make sure you understand blue attacks

Go on the date with Papyrus. Go to his house and he'll ask you something like: "Are you ready to start the date?" and choose yes. After your date, he'll give you his phone number.

Spare the Mad Dummy. This can be done by luring the attacks so that they injure him instead. However, when Mad Dummy starts using rocket attacks, you don't need to hurt him with them.

Go visit Napstablook's house. It is the house on the left next to the Snail Races. This step isn't necessary to complete the Pacifist Run, but he has a pretty cool house.

Make your way to Undyne's Cave. This can take a while due to her excessive spear attacks, but once you get there it'll pay off.

Spare Undyne. Then Flee when your heart turns red. During this, you'll have to fend off her spear attacks, but they are fairly easy to memorize.

Go up through the cave and keep Fleeing. After every battle, Undyne will become more and more tired. Just keep on fleeing after your heart turns red and run along the path to the Hotlands.

Save Undyne when you reach the Hotlands. She will collapse on the ground because of her too-hot armor. After that, go to the water cooler and grab a glass of water, then pour it on Undyne's head.

Become friends with Undyne. You do this by walking all the way back to Waterfall and entering the house in the entrance right of Napstablook's. Papyrus will be waiting to "introduce" you to Undyne. After that, it's pretty straightforward.

Don't kill any monsters. You'd be surprised at how many people forget.

Go through Alphys' Lab. When you enter the Hotlands you'll notice a large, white building on the right of the screen, which you should enter. Mettaton's quiz is very easy; Alphys will give you all the answers. If she doesn't, then that means any answer will qualify. However, even if you get every question wrong, you won't die.

Progress through assorted puzzles, games, etc. There are lots of puzzles and minigames made by Alphys and Mettaton that you have to go through to reach the CORE, but they are pretty self-explanatory.

• If you are having trouble with any of the puzzles, Alphys will usually give you a phone call explaining the rules and possible solutions to your current puzzle.

Buy healing items at MTT Resort. After entering, looking at one of the trash cans can give you a healing item.

Wait in the room. Normally, two paths have to be completed to access the final part. However, you can skip both if you wait in the room with the forcefield until it disappears.

Fight Mettaton EX. Boost the ratings to 10,000, or if his limbs are still intact, 12,000 to win the fight.

• To boost your ratings, you can type certain words in the essay, eat foods from MTT Resort or the trash can from earlier, wear different armor that hasn't been equipped before in the battle (not the entire game, but single battle), and use certain ACTs.

Don't do anything. However, if you haven't completed a Neutral Route before, you'll have to fight Asgore, and then Flowey.

Beat Asgore and Flowey. This will close the game, and after opening, a fake intro plays.

• Fight Flowey. It is actually much easier than it seems. After you get through his first few rounds of attacks, the souls of the other Fallen children will aid/heal you in defeating Flowey.
• Spare Flowey. Sparing him 13 times will earn you a hint on getting the Pacifist Ending after getting Sans' call.
• Again, if you have already completed a Neutral run, skip this step.

To beat Asgore, TALK to him 3 times, then FIGHT him. (Will not effect pacifist)

Answer your phone. This time, you will probably get a call from Undyne. Afterward, backtrack to Papyrus' house.

Trigger the dating event with Alphys. After that, go to Alphys' Lab in the Hotlands again.

Go into the "bathroom". It is actually an elevator, that breaks and makes you fall to the True Lab.

Spare all the amalgamates. Put all the keys in the holes. There are four keys scattered throughout the True Lab.

Get the Red Key. Spare Memoryhead.

Get the Yellow Key. Examine the bed in the center row and right column, in the bedroom. You can find this directly to the right of the elevator room.

Get the Blue Key. Spare Snowdrake's Mother.

Get the Green Key. Examine the shower curtain.

Take the elevator. You'll go to New Home, and the elevator will be sealed by vines.

Go to the barrier. Trigger the Asgore fight.

Prepare for the final battle. Toriel will appear and launch Asgore with a fireball, followed by all the other main monsters. After a cutscene, you will be taken to the final battle.

Do anything on your turn during the first phase. Continuously use Struggle in the next phase.

Save your friends. After a cutscene, you need to SAVE all your friends by using any ACT at certain times.

Backtrack to any area in the Underground. Talk with the NPC's with different dialogues.