True Pacifist Route

The True Pacifist Route leads to one of the Endings of Undertale. This ending can be considered to be the "true ending" to Undertale, leading to a happy conclusion and the complete credits.

Method
Upon completing the Neutral Route, the protagonist will receive a phone call post credits that will provide a hint to the next requirement for getting the True Pacifist Ending. If Flowey is spared, he will appear afterward and elaborate on the hint, but if he is killed, the game immediately returns to the title screen. The protagonist can then reload their save file to fulfill any missing requirements as long as they have not gained any EXP/LV; otherwise, they must reset their game. Note that killing Asgore or Flowey does not acquire any EXP, and the protagonist cannot SAVE after fighting them anyway.

The list of requirements are as follows:
 * 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). FIGHTing monsters or destroying the Dummy in the ruins does not ruin this route, as they do not accrue EXP.
 * 2) After fighting Papyrus, visit his house and complete the date/hangout with him.
 * 3) After Undyne passes out from pursuing the protagonist, 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, then complete the "cooking lesson" and faux battle with Undyne.
 * 5) Go through Hotland and the Core; spare Mettaton EX.
 * 6) Complete the Neutral Route (if it has not been completed in a previous run) to begin the next series of events.
 * 7) Backtrack out of the Core towards the MTT Resort, to the bridge connecting the two places. There, Undyne will call the protagonist, asking to deliver a letter for her.
 * 8) Talk to Undyne outside of Papyrus's house to receive Undyne's Letter.
 * 9) *If the protagonist had reached the bridge without completing the "date" with Undyne, then backtracked to complete such, she will give them the letter after the hangout, unless their inventory is full in which they will have to receive it from her at Papyrus's house.
 * 10) *If the protagonist discards the letter, they can return to Undyne to receive Undyne's Letter EX, which cannot be discarded.
 * 11) Put the letter under the door of Alphys's Lab and complete the "date" with her.
 * 12) Upon leaving the Garbage Dump (where befriending Alphys ends), Papyrus will call and tell the protagonist to meet Alphys at her lab. Return to Alphys's Lab to find a note leading to her "bathroom" containing the True Lab.
 * 13) 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. Head back to the elevator near the generator room to be warped to New Home, with the exit vined off. This signals that the True Pacifist Ending will occur.

Completing Friendships

 * If Papyrus has not been befriended, backtrack to his house in Snowdin. He will be standing outside. Interact with him to initiate the date/hangout.


 * If Undyne has not been befriended, backtrack to her house in Waterfall. Papyrus will be standing outside. Interact with him to initiate the hangout.

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.

If Undyne was befriended in a Pacifist Neutral run, she will give the protagonist the letter immediately after befriending her. She will say that Papyrus suggested giving the protagonist the letter earlier, but that Undyne still hated them at the time.

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's date, the two go to the Garbage Dump. 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, watching anime and reading manga. Undyne expresses her support for Alphys, sending her off to train with Papyrus. She then confronts the protagonist about anime, asking if it is real. Regardless of the protagonist's response, Undyne will thank them, and the event will end.

After the date, Papyrus calls the protagonist and very specifically instructs them to go to Alphys's lab, citing no reason other than a 'good feeling' he claims to have about doing so. (This may be because Flowey told him to tell the protagonist, but didn't want the protagonist to know.)

True Lab
Arriving at the lab, the protagonist 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's bathroom to discover the truth, in case Alphys does not return.

The protagonist enters the door, which actually leads to an elevator. They take the elevator down to the True Lab, but it loses power and malfunctions, leaving them stuck in the lab. Exploring the lab, they find logs from long ago, detailing Alphys's experiments with SOULs and Determination to restore monsters from death. Injecting determination into the dead monsters successfully restored them to life, but they melted and turned into Amalgamates - fusions of several monsters. Several of these amalgamates attack the protagonist, but the protagonist spares 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. Some Amalgamates appear, but Alphys saves the protagonist from them, explaining that they were aggressive because they were hungry. She further explains that the monsters' bodies could not handle the determination, and thus melted together. Alphys thanks the protagonist for their support, and declares to the Amalgamates that they will return home.

Alphys leaves the true lab, and the protagonist returns to one of the elevators. They receive a call from someone they do not recognize. The voice, implied to be Asriel (Flowey mimicking their 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' and that they will 'see you soon.'

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 protagonist confronts Asgore. As they prepare to fight, Toriel arrives, incapacitates Asgore, and consoles the protagonist. 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. To Alphys's surprise, Papyrus reveals that a 'tiny flower' helped him call everybody to the Throne Room.

Flowey arrives and traps the other monsters in vines. He then thanks the protagonist for bringing them all to one spot, saying that he absorbed the humans' souls while the protagonist's friends were chatting, claiming he will absorb them all as well to achieve his 'real form.' He then reveals his true motive for causing the protagonist so much trouble - to keep them playing the 'game' so that they can 'play' with him forever. He gives the protagonist 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 protagonist's friends protect them from Flowey's attacks while both healing them and encouraging them to win. Shortly afterward, many of the monsters that the protagonist spared throughout the game arrive, healing the protagonist and seemingly putting Flowey off guard. However, he suddenly laughs at them for their 'stupidity' and absorbs their souls.

The protagonist sees that Flowey is now a young monster in a striped shirt, similar in appearance to Toriel and Asgore. He calls the protagonist by the Fallen Human's name, then transforms into an older, more powerful looking monster, introducing himself as Asriel Dreemurr. Asriel attacks the protagonist; if the protagonist's HP reaches 0 and their soul breaks, instead of restarting the game, the message "But it refused" appears and the soul reforms through determination and fully restores its health. After this happens once, the soul will simply reform and return to the battle.

After a certain amount of turns, Asriel attacks with an attack called "Hyper Goner," which resembles a goat skull (similar to the DT Extractor in the True Lab), sucking in bullets (and the SOUL) until the screen turns to white. The protagonist's HP will be left at one after this attack if they get hit. Surprised at the protagonist's high determination, Asriel reveals that he was only using a fraction of his real power.

Asriel then transforms into a more powerful form of himself, threatening to reset the world's timeline, restraining the protagonist and preventing them from doing anything except struggling. The protagonist realizes that while they cannot save their game, they can save the monsters' souls, transforming the ACT option into the SAVE option. They reach out to their friends' souls (which are trapped in Asriel's soul) and bring them back by restoring their memories. After saving all their friends, the protagonist begins to save Asriel himself and see memories of him befriending the first human. The protagonist's friends' SOULs resonate within Asriel; he begins to cry, and his fire attacks start avoiding the protagonist's soul. He then begs the protagonist (referring to them as the Fallen Human's name) 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 revealed to be "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 not to forgive him for everything that happened, but they can choose to anyway, surprising Asriel. Using the six humans' souls and the monsters' souls along with their determination, Asriel destroys the barrier.

Asriel returns the souls and tells Frisk that without everyone's souls, he will eventually return to 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.

Epilogue
Frisk wakes up in the room behind the Throne Room, much to the relief of their friends, who all know Frisk's name but 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 are ready.

Frisk can go through the Underground and talk to NPCs, who will express their feelings about being free at last. In the beginning area with the golden flowers, Asriel can be found tending the flowers and can be talked to.

The End
Leaving for the surface will end the game.

Frisk and their friends watch the sunset on the surface. Thinking about the future, Asgore asks Frisk if they would act as the monsters' ambassador to the humans.

Papyrus leaves to make a good first impression on the humans, having declared himself the mascot or ambassador depending on Frisk's decision. Sans retreats to the Underground (presumably to a shortcut) to ensure he does not have to check on Papyrus, while Undyne, Alphys, and Asgore leave to help Papyrus.

With everybody else gone, Toriel asks Frisk what they will do now. The protagonist can choose to stay with Toriel or not, and their decision will affect the post-credits scene.

After Toriel and Frisk leave the screen, the True Pacifist Ending Credits will start. The names of all the monsters encountered in the game (except Jerry and secret bosses So Sorry and Glyde) will scroll down (depending on the areas in which they reside), and then a final bullet hell introduced by Asriel will be initiated. This section includes the names of Kickstarter backers, and if all of the names are dodged, the Mysterious Door will be unlocked.

After the credits, if Frisk is staying with Toriel, another scene will play, showing them in their new home on the surface. If Frisk parted ways with the monsters, then a photo of Frisk and their monster friends is shown. The Annoying Dog appears after some time on the ending screen. The player can do nothing except quit the game.

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 asks that if they still wish to reset the game, the player needs to erase Flowey's own memories as well. 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 at the beginning of the game.

The game's menu has none of Frisk's friends, as they have all left. The menu music is a significantly slowed down version, giving an ominous tone. The player can load their save, which takes place before the ending, and they can replay the credits (and the special thanks) from there (though Asriel will not appear). The menu's reset option is replaced with 'True Reset'; this allows the player to rename the Fallen Human. Choosing a name resets the game and removes everybody's (including Frisk's and Flowey's) memories.

Post-Genocide Game
After the protagonist successfully completes a Genocide Route and restores the world after erasing it, several things will be significantly altered.
 * If the protagonist chose "I have places to go" when Toriel asks, the picture in the credits will show all of the characters' faces crossed out, and the protagonist will be replaced with the sprite of the First Human, implying that all of them have been killed.
 * If the protagonist chose "I wish to stay with you", after Toriel leaves the pie in the room, within four seconds, Frisk will turn in the bed to face the screen, only to reveal Chara's face with red eyes. A low pitched version of Flowey's laugh will play, and the screen cuts to black.
 * The Annoying Dog will not appear to sleep under THE END after the credits.

Trivia
Ruta Pacifista Verdadera Wahre Pazifisten-Route Истинный Пацифистский Путь Істинний Пацифістський Шлях 完美路線
 * It is only in the True Pacifist Route that the player learns the protagonist's name, Frisk.
 * Naming the fallen human "Frisk" at the beginning of the game will activate Hard Mode.
 * If a True Pacifist Ending has been completed before, then Sans's Workshop will contain a photo of "the protagonist" standing with Sans and all [their] friends". This is likely the picture shown at the end if the protagonist chooses not to stay with Toriel.
 * There was a glitch in which the protagonist could fight and kill enemies in the Core before receiving Undyne's Letter. This glitch allowed the protagonist to reach this ending even after gaining EXP and LOVE. It has since been patched.
 * If the spacebar is pressed when Alphys and Undyne are standing close to each other, after the Faux Asgore battle, then it will start their kissing animation very early.