True Pacifist Route

The True Pacifist Route leads to the objectively "best" Ending of Undertale.

Method
Upon completing the Neutral Route, the player will receive a phone call post credits that will provide a hint to the next requirement in getting the True Pacifist Ending. If Flowey is spared, he will appear afterward and elaborate on the hint. The player can then reload the save file or restart the game to fulfill any missing requirements.

The list of requirements are as follows: Exiting the True Lab will transport the player to New Home with the exit vined off. This signals that the Pacifist Ending will occur.
 * 1) 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)
 * 2) After fighting Papyrus, visit his house and complete the date/hangout with him.
 * 3) After Undyne passes out pursuing you, get a cup of water from the nearby water cooler and pour it on her.
 * 4) Return to Waterfall and go to Undyne's house. Papyrus will be waiting outside. Talk to him to enter the house.
 * 5) Complete the "cooking lesson" and faux battle with Undyne.
 * 6) Play through the game as you would normally in the neutral route (except with no killing) until you get to the Core.
 * 7) Complete the core and spare Mettaton EX.
 * 8) After that, backtrack out of the Core towards the MTT Resort, where you entered the Core from.
 * 9) Along the bridge connecting the resort to the Core, Undyne will call you asking you to deliver a letter for her, telling you she'll be waiting outside Papyrus' house where she'll give you the letter upon talking to her. (reminder: this event will only happen if you have already completed the Neutral Route, though you should be able to beat the game on this route during a Pacifist Run and then load your save before the final boss.)
 * 10) Put the letter under the door of Alphys' Lab and complete the "date" with her.
 * 11) Return to Alphys' Lab and enter the "bathroom" to access the True Lab.
 * 12) Find the four keys scattered throughout the True Lab and put them in their proper slots to open the generator room. Turn on the generator to trigger a conversation with Alphys.

If the Neutral Route was completed without killing any enemies, the player has the option of restoring from the final save point and backtracking to complete any missing friendships (if any); once this is completed, point Papyrus will call you and encourage you to visit Alphys, leading to the True Lab scenario.

Befriending Alphys
At the entrance to the Core, Undyne calls the protagonist (at the behest of Papyrus) to ask them to deliver her letter to Alphys. After delivering the letter to Alphys, she mistakes the letter as being from the protagonist themself, and goes on a date with them.

During Alphys' date, the two go to the garbage dump near the Waterfall. They bump into Undyne, who decided to deliver the letter herself. Alphys tells Undyne some truths about her 'scientific' work, which was just her lazing around and watching anime and manga. Undyne expresses her support for Alphys, sending her off to train with Papyrus. She then confronts the player about anime, asking if it is real.

After the date, Papyrus calls the protagonist and very specifically instructs them to go to Alphys' lab, citing no reason other than a 'good feeling' he claims to have about doing so.

True Lab
Arriving at the lab, the player finds a note from Alphys, in which she declares she will 'face [her] own mistakes'. The note tells the reader to enter what was previously thought to be Alphys' bathroom to discover the truth, in case Alphys does not return.

The protagonist enters the door, discovering it to be an elevator. They take the elevator down to the True Lab, but it loses power, leaving them stuck in the lab. Exploring the lab, they find logs from long ago, detailing Alphys' experiments with souls and determination to restore monsters from death. Injecting determination into the dead monsters successfully restored them to life, but they turned into amalgamates - fusions of several monsters. Several of these amalgamates attack the protagonist, but they spare them. The protagonist sees the determination extractor, resembling part of Flowey's form during his boss battle in the Neutral Route, and they learn that Alphys did some experiments with golden flowers, which produced a single living specimen who escaped.

The protagonist switches the power on, restoring power to the elevators. Alphys appears and saves them from some amalgamates, who she explains were hungry, resulting in their aggression. She further explains that the monsters' bodies couldn't handle the determination and melted together. Alphys thanks the player for their support, and declares to the amalgamates that they will return home.

Alphys leaves the true lab, and the player returns to one of the elevators. They receive a call from someone they do not recognise. The voice calls the protagonist by the name given to the Fallen Human at the beginning of the game, and tells them that 'everything has fallen into place'. The elevator appears to malfunction, and lands the protagonist at the entrance to New Home. Vines jam the elevator shut.

The Barrier
At the barrier, the player confronts Asgore. As they prepare to fight, Toriel arrives, incapacitates Asgore, and consoles the player. After reprimanding Asgore for his actions, even going as far as to give him a simpler way to retrieve human souls, she is shortly later followed by Sans, Papyrus, Undyne, and Alphys who all begin to chat. Papyrus reveals that a 'tiny flower' helped him call everybody to the Throne Room, surprising Alphys.

Flowey arrives and traps the other monsters in vines. He absorbed the humans' souls while the protagonist's friends were chatting, thanks the player for bringing them all to one spot, claiming he will absorb them as well to achieve his 'real form'. He then reveals his true motive for causing the player so much trouble - to keep them playing the 'game' so that they can 'play' with him forever. He gives the player a chance to face him, promising to return everyone's souls and break the barrier if they win. As if to deny them even this, he plans to keep them trapped before mercilessly attacking them, saying he is willing to "kill them 1,000,000 times" to prove his point. Before delivering the final blow, the player's friends protect them from Flowey's attacks while both healing them and encouraging them to win. Shortly afterwords, many of the monsters that the player spared through out the game arrive, healing the player and seemingly putting Flowey off guard. However, he suddenly laughs at them for their 'stupidity' and absorbs their souls.

The player sees that Flowey is now a young monster in a striped shirt similar in appearance to Toriel. He calls the player by the Fallen Human's name, then transforms into an older, more powerful looking monster, introducing himself as Asriel Dreemurr. Asriel attacks the player's soul; if the player's HP reaches 0 and their soul breaks, it reforms through determination and fully restores its health.

Asriel changes into an even more powerful form, restraining the protagonist and preventing them from doing anything except struggling. The protagonist realizes that while they can't save their game, they can save the monsters' souls. They reach out to their friends' souls (which are trapped in Asriel's soul) and restore their memories. After saving all their friends, they begin to save Asriel himself and see memories of the first human befriending Asriel. While the player continues to save Asriel, he begins to cry and his fire attacks start avoiding the player's soul. He then begs the player to let him win and attacks them with his strongest attack. The protagonist survives with their HP reduced to an extremely small decimal value and saves Asriel, returning him to his form as a child.

Asriel apologizes to the protagonist. He tells them that the first human has been gone for a long time, and asks them for their real name, which is Frisk. Asriel tells Frisk that the monsters' souls allowed him to feel their emotions and desires - their love for Frisk and their desire for freedom, respectively. He expects Frisk to not forgive him for everything that happened, but the player can choose to anyway, surprising Asriel. Using the six humans' souls and the monsters' souls along with their determination, he destroys the barrier.

Asriel returns the souls, and tells Frisk that without everyone's souls, he will eventually return to being a flower. He asks them to forget about him and leaves, but Frisk has the option to console Asriel, causing Frisk to approach and hug him for a heartwarming scene.

Reunited
Frisk wakes up in the room behind the Throne Room, much to the relief of their friends, who all know Frisk's name but they do not remember much about what just happened. Toriel encourages Frisk to do as they wish, whether that be heading for the surface or going for a walk Underground to see the rest of their friends. The monsters inform Frisk that they will follow them to the surface when they're ready.

If the player takes Frisk to the beginning area of the game, where the golden flowers broke their fall, then Asriel will be there tending to the flowers. Asriel will be only there if the player hasn't left the Underground before.

The End
Leaving for the surface will end the game and will give the player two more choices: The second choice determines the post credits scene.
 * 1) Become the monsters' ambassador to the humans (or not)
 * 2) Stay with Toriel (or not)

During the credits, the player gains control of their soul during the special thanks, which are dedicated to the game's Kickstarter backers, and are challenged with dodging the backers' names. If they haven't touched a single name, then the mysterious door in Snowdin Forest will open.

After the credits, if the player reopens the game, Flowey speaks to the player, telling them that 'there's nothing left to worry about'. He points out that the player is the only threat left, as they have the power to reset everything. He then asks them not to reset the game, which would return everybody to when they were trapped underground, and asks the player to let them go. Flowey then lets the player know his memories will disappear as well if they still wish to reset the game. Believing they might have heard this a hundred times already, he says his goodbyes to the player, calling them by the name given to the First Human.

The game's menu has none of Frisk's friends, as they have all left. The menu's happy music is replaced with the ambient noise of machinery. The player can load their save, which takes place before the ending, and they can replay the credits (and the special thanks) from there. The menu's reset option is replaced with 'True Reset'; this resets the game as well as everybody's memories, including Frisk's and Flowey's, and allows the player to rename the Fallen Human.

Trivia

 * It is only in the Pacifist Route that the player learns the protagonist's name, Frisk.
 * Examining mirrors after the final battle gives the description 'Still just you, Frisk.'
 * Naming your character 'Frisk' at the beginning of the game will activate Hard mode.


 * After saving Asriel, the player can go to Sans' back room and find a photo 'of [Frisk] standing with Sans and all your friends.' This photo is implied to be the one the player sees if they decide not to stay with Toriel.