Neutral Route

One of the three primary routes in Undertale, and the one that the majority of players will see on their first playthrough. It is in this route that the final boss is Flowey. It is necesary to complete this route to achieve a true Pacifist Ending.

Method
The Neutral Ending is what a player will see if they do not complete the Genocide Route or the Pacifist Route. To see it: The additional sidequests for the Pacifist Route only open up once the Neutral Ending has been seen at least once. This means that players who have spared every enemy on their first playthrough will still see the Neutral Ending on their first run through the game.
 * do not complete all of the sidequests involving Alphys (as this leads to a Pacifist Ending).

Neutral Ending Possibilities
In all endings, Asgore dies, Flowey is defeated, the six human souls are lost, the protagonist escapes the Underground and the monsters remain trapped behind The Barrier. There are many versions of this ending all stating with Sans's calling and leaving a message telling the player of the Underground's fate. However, how things turn out after the player leaves depends on who the player befriended, spared, and killed.

Pre-Pacifist, Papyrus and Undyne befriended
This is an ending where the player does not kill any enemies or bosses (Asgore/Flowey doesn't count), and befriends Papyrus and Undyne. In it, Toriel returns to rule the underground, and enstates a policy of treating humans as friends. Papyrus is appointed captain of the royal guard, although he doesn't do any actual fighting. Undyne begins to work as Alphys's lab assistant, attempting to figure a way out of the Underground. Additionally, Undyne becomes a gym teacher at Toriel's new school.

This is identical to any Neutral Ending, but after the credits, receives a phone call guiding them to befriend Alphys. If Flowey is left alive he hints towards this, allowing the player to reload the file and progress towards a Pacifist Ending.

Papyrus, Undyne and Alphys befriended
This is an ending where the player does not kill any enemies or bosses (except Asgore or Flowey), and befriends Papyrus, Undyne and Alphys, but does not complete the True Lab sidequest. It is similar to the Pre-pacifist ending: Toriel returns to rule the underground, and enstates a policy of treating humans as friends. Papyrus is appointed captain of the royal guard, although he doesn't do any actual fighting. Undyne begins to work as Alphys's lab assistant, attempting to figure a way out of the Underground. Additionally, Undyne becomes a gym teacher at Toriel's new school.

Differently, Alphys talks on the phone. She tells them that she's been working on alternative ways of leaving the Underground (with no idea of what to do), and that Toriel has been checking if she's working, unlike Asgore.

Friendless Pre-Pacifist or Only Papyrus Befriended.
This is an ending where the player does not kill any enemies or bosses (Asgore/Flowey dont count), but doesn't befriend Papyrus and Undyne. In it, Toriel returns to rule the Underground, and enstates a policy of treating humans as friends. The royal guard is disbanded, but Papyrus is fine. Undyne loses her job and home, and goes to live with Papyrus and Sans. Sans gives Undyne a job at his illegal hotdog stand. Undyne plots to cross the barrier and "beat you up," although Papyrus thinks it's impossible.

Only Toriel Slain, Only Papyrus Befriended.
Undyne becomes the ruler of the underground, and continues with the plan to destroy all humans and wage war on humanity. She's militarized the underground, and plans to personally destroy the player. She appointed Papyrus "The Most Important Royal Position," which is basically a figurehead position, but he's proud of it.

Papyrus Slain, Toriel Befriended (varies depending on Undyne's fate)
Toriel becomes the ruler of the underground, and attempts to institute a policy of treating humans as friends. However, this was met with dislike from the monsters, and a revolution is formed, forcing her to step down. If Undyne is alive, she personally spearheads the revolution and throws the queen back into the ruins before taking the throne herself. However, Sans accompanied the queen back to the ruins. He doesn't have the heart to tell her that you killed Papyrus. If Undyne is the new queen (or empress in her case), Sans warns you to watch yourself as Undyne will have her revenge should she find a way to break the barrier. Otherwise, he finishes his call by telling you to never come back as you are no longer welcome. Mettaton and Alphys are not mentioned. If you killed off all the random encounters this is also mentioned.

Only Undyne Slain
Toriel returns to rule the underground, and enstates a policy of treating humans as friends. However, people didn't like that policy, and a rebellion was started to overthrow the queen. Toriel gave up peacefully, and returned to the ruins. The underground became an uneasy anarchy, but it's alright. Sans and Papyrus visit Toriel and keep her company.

Mettaton Slain, Papyrus and Undyne Befriended
Undyne is currently living with Papyrus and Sans. She's fallen into depression, devastated over the deaths of Asgore and Alphys. She absolutely despises you for making friends with her, then betraying her trust by killing Mettaton.

Papyrus and Undyne befriended, monster(s) slain, Mettaton spared
Toriel returns to rule the underground, and enstates a policy of treating humans as friends. Undyne lives with Papyrus and Sans. She despises you for making friends with her then killing someone, and blames their death on herself as she befriended you. Papyrus warns that if you returned, Undyne would ignore Toriel's policy to destroy you.

Only Mettaton Slain, Papyrus Befriended
Toriel becomes the ruler of the underground, and attempts to institute a policy of treating humans as friends. However, this was met with dislike from the monsters, and Undyne spearheads a revolution, and throws the queen back into the ruins. Sans and Papyrus visit Toriel and keep her company.

Only Toriel and Undyne Slain
Mettaton becomes ruler of the underground, and establishes a dystopian society in which he brainwashes the entire underground with his television show. Sans and Papyrus become his agents. Alphys disappears.

Only Toriel, Undyne, Mettaton and Asgore Slain
By process of elimination, Papyrus became the ruler of the underground. Papyrus is told by Sans that Asgore, Alphys, Mettaton and Undyne are on vacation. The underground is generally improving: productivity is up, Papyrus cooks everyone spaghetti. The human policy instituted is that humans cannot be judged to be all good or all bad. Either way, they get puzzles.

Bastille
This ending occurs when all bosses are slain, but all other monsters are spared. A small, white dog becomes president of the Underground. Strangely, this is the best life for everyone. Oddly enough, he does not mention you killing Papyrus.

Near Genocide
This ending occurs when you back out of a genocide route at the last possible moment, leading to Alphys ruling the underground. Alphys finally gains some self confidence, but regrets not killing you when she had the chance.

No Mercy
This endings occurs when you kill every boss you encounter plus at least one enemy, but not enough to trigger the genocide route. The underground is leaderless, and less crowded. The mood is somber, and people feel as if they're just going to die in the Underground, trapped in the dark. Sans wishes for you to go to hell. If only a few non-boss monster were killed, Sans says "see ya" instead.

???
Embedded in the game data is an ending for when none of the above occurs; Sans calls the player and says the following:
 * i'll be honest.
 * i have no idea what happened for you to get here.
 * this is actually some sort of error-handling message.
 * so, if you're getting this ending...
 * tell whoever made the game, okay?
 * they'll fix it, or if it's a novel situation...
 * they might even add another ending to the game.
 * chances are, though...
 * you're just a dirty hacker, aren't you?
 * yeah, get outta here.

Other factors
When ending happens while in possession of a snow piece, Sans will inform that the player has made a snowman very happy.

If finishing the route without eating food and/or without taking off the bandage, Sans will comment on it.