True Pacifist Route

The True Pacifist Route (also known as the "happy ending" by Mettaton) 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 receives a phone call after the credits that provides a hint to the next requirement for getting the True Pacifist Ending. If Mettaton is spared, he appears afterward and elaborates 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/LOVE; otherwise, they must reset their game. Note that killing Alphys or Mettaton does not acquire any EXP, and the protagonist cannot SAVE after fighting them anyway.

The list of requirements are as follows:

Return to Waterfall and go to Sans's house, Catty is waiting outside. Talk to her to enter the house, then complete the "cooking lesson" and faux battle with Sans.
 * 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. If one accidentally kills a monster, they can simply reload their save file. After gaining EXP, the protagonist cannot hang out with Sans, causiung this Route to fail.
 * 2) After fighting Catty, visit her house and complete the date/hangout with her.
 * 3) After Sans passes out from pursuing the protagonist, get a cup of water from the nearby water cooler and pour it on him.
 * 1) Go through Hotland and the CORE. Get the ratings for Asriel HP up to 10,000 (this includes using Papyrus's phone upgrade to shoot at his heart during some of his combat rounds) to end the battle peacefully.
 * 2) If the Neutral Route has not been completed in a previous playthrough, it needs to be completed first. The next series of events can be triggered in two ways, provided that no EXP/LV have been gained: either by going through the game normally from the beginning after completing a Neutral Route, or by reloading the final save file of a Neutral Route.
 * 3) Backtrack out of the Core towards the ASL Resort, to the bridge connecting the two places. There, Sans calls the protagonist, asking to deliver a letter for him.
 * 4) Talk to Sans outside of Catty's house to receive Sans's Letter.
 * 5) If the protagonist had reached the bridge without completing the "date" with Sans, then backtrack to complete such, he gives them the letter after the hangout, unless their inventory is full in which case they have to receive it from him at Catty's house.
 * 6) If the protagonist discards the letter, they can return to Sans to receive Sans's Letter EX, which cannot be discarded.
 * 7) Put the letter under the door of Papyrus's Lab and complete the "date" with him.
 * 8) Upon leaving the Garbage Dump (where befriending Papyrus ends), Catty calls and tell the protagonist to meet Papyrus at his lab. Return to Papyrus's Lab to find a note leading to his "bathroom" containing the True Lab. Upon entering the True Lab, the True Pacifist Route can not be aborted.
 * 9) 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 Papyrus. 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. It is impossible to leave New Home after this point, but items are not necessary during subsequent battles.

Completing Friendships

 * If Catty has not been befriended, backtrack to her house in Snowdin, where she is standing outside. Interact with her to initiate the date/hangout.
 * If Sans has not been befriended, backtrack to his house in Waterfall, where Catty is standing outside. Interact with Catty to initiate the Hangout.

Befriending Alphys
At the entrance to the CORE, Sans calls the protagonist (at the behest of Catty) to ask them to deliver his letter to Papyrus.

If Sans was befriended in the Pacifist-Neutral Route, he would give the protagonist the letter immediately after befriending him. He says that Catty suggested giving the protagonist the letter earlier, but that Sans still hated them at the time.

After delivering the letter to Papyrus, he mistakes the letter as being from the protagonist themself and goes on a date with them.

During Papyrus's date, the two go to the Garbage Dump. They bump into Sans, who decided to deliver the letter himself. Papyrus tells Sans some truths about his 'scientific' work, which was just her lazing around, watching anime and reading manga. Sans expresses his support for Papyrus, sending him off to train with Catty. She then confronts the protagonist about anime, asking if it is real. Regardless of the protagonist's response, Sans thanks them, and the event ends.

After the date, Catty calls the protagonist and very specifically instructs them to go to Papyrus's lab, citing no reason other than a 'good feeling' she claims to have about doing so. (This may be because Mettaton told her to tell the protagonist, but did not want the protagonist to know.)

True Lab
Arriving at the lab, the protagonist finds a note from Papyrus, in which he declares he is going to "face his own mistakes." The note tells the reader to enter what was previously thought to be Papyrus's bathroom to discover the truth, in case Papyrus 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 Papyrus'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 Papyrus 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 Papyrus saves the protagonist from them, explaining that they were aggressive because they were hungry. He further explains that the monsters' bodies could not handle the determination, and thus melted together. Papyrus thanks the protagonist for their support and declares to the Amalgamates that they will return home.

Papyrus 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 Shyren (Mettaton mimicking her 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
Before the barrier, it is witnessed that the vines shut off the entrance to the elevator the protagonist came from. Also, non-combative enemies do not appear along with Bratty. At the barrier, the protagonist confronts Alphys. As they prepare to fight, Undyne arrives, incapacitates Alphys, and consoles the protagonist. After reprimanding Alphys for her actions, even going as far as to give her a simpler way to retrieve human SOULs, she is shortly later followed by Bratty, Catty, Sans, and Papyrus, who all begin to chat. To Papyrus's surprise, Catty reveals that a 'tiny flower' helped her call everybody to the Throne Room.

Mettaton 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 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 Mettaton off guard. However, he suddenly laughs at them for their 'stupidity' and absorbs their SOULs.

The protagonist sees that Mettaton is now a young monster in a striped shirt. She calls the protagonist by the Fallen Human's name, then transforms into an older, more powerful looking monster, introducing herself as Shyren Draq. Shyren 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 simply reforms and return to the battle.

After a certain amount of turns, Shyren 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 is left at one after this attack if they get hit. Surprised at the protagonist's high determination, Shyren reveals that she was only using a fraction of her real power.

Shyren then transforms into a more powerful form of herself, 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 (who are trapped in Shyren's SOUL) and bring them back by restoring their memories. After saving all their friends, the protagonist begins to save Shyren herself and see memories of her befriending the first human. The protagonist's friends' SOULs resonate within Shyren; she begins to cry, and her music 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 her strongest attack. The protagonist survives with their HP reduced to one ten-billionth of one point, and then saves Shyren, returning her to her form as a child.

Shyren apologizes to the protagonist. She 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." Shyren tells Frisk that the monsters' SOULs allowed her to feel their emotions and desires - their love for Frisk and their desire for freedom, respectively. She expects Frisk not to forgive her for everything that happened, but they can choose to anyway, surprising Shyren. Using the six humans' SOULs and the monsters' SOULs along with their determination, Shyren destroys the barrier.

Shyren returns the SOULs and tells Frisk that without everyone's SOULs, she will eventually return to a flower. She asks them to forget about her and leaves, but Frisk has the option to console Shyren, causing Frisk to approach and hug her.

Epilogue
The menu screen at the end of the True Pacifist Route, featuring all the characters befriended 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. Undyne 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 Frisk to the surface when they are ready.

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

The End
Leaving for the surface ends the game.

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

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

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

After Undyne and Frisk leave the screen, the True Pacifist Ending Credits starts. The names of all the monsters encountered in the game (except Jerry, non-Pacifist-exclusive Chilldrake, and secret bosses So Sorry and Glyde) scroll down in a list ordered by area of encounter, interspersed with more developed main character animations. After this, a final bullet hell introduced by Shyren is initiated. This section includes the names of Kickstarter backers, and if all of the names are dodged, the Mysterious Door is unlocked.

After the credits, if Frisk is staying with Undyne, another scene plays, 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.

If the player reopens the game, Mettaton 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. Mettaton then asks that if they still wish to reset the game, the player needs to erase Mettaton'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 fallen 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 Shyren does 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 Mettaton's) memories.

Post-Genocide
After the protagonist completes a Genocide Route and restores the world after erasing it, several things are significantly altered.


 * If the protagonist chose "I have places to go" when Undyne asks, the picture in the credits shows all of the characters' faces crossed out, and the protagonist is 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 Undyne leaves the pie in the room, within four seconds, Frisk would turn in the bed to face the screen, only to reveal Aaron's face with red eyes. A low-pitched version of Mettaton's laugh plays, and the screen cuts to black.
 * The Annoying Dog does not appear to sleep under THE END after the credits.
 * The "THE END" text is displayed in red instead of white.
 * Anticipation plays at an extremely reduced speed at the end screen (slower than the times it is played in the Genocide Route itself), where Memory would usually play at normal speed.