|“||i know your type. you're, uh, very determined, aren't you? you'll never give up, even if there's, uh... absolutely NO benefit to persevering whatsoever. if I can make that clear. no matter what, you'll just keep going. not out of any desire for good or evil... but just because you think you can. and because you "can"... ... you "have to".||„|
|~ Sans describing the Anomaly's amoral motives.|
|“||But. You and I are not the same, are we? This SOUL resonates with a strange feeling. There is a reason you continue to recreate this world. There is a reason you continue to destroy it. You. You are wracked with a perverted sentimentality. Hmm. I cannot understand these feelings any more. Despite this. I feel obligated to suggest. Should you choose to create this world once more. Another path would be better suited. Now, partner. Let us send this world back into the abyss.||„|
|~ Chara to the player if they succeed in a second Genocide run.|
The Anomaly is the overarching protagonist of Toby Fox's Undertale and Deltarune. They are simply the player, and in-universe they serve as a driving force throughout the game, aiding Frisk on their journey throughout the Underground and manipulating the Underground itself. Seeing as they are the player, their alignment depends on what route they take.
As Undertale is a game that's very big on meta-commentary, the player themselves is just as much a character as anyone else inside the game, and the most powerful being in the context of the game as well. In any playthrough, the player is the one controlling Frisk and the being who makes every decision. At any moment the Anomaly can turn back time to the point where Frisk fell into the Underground, the only ones capable of overriding this power being Flowey after absorbing human souls and Chara after the latter reaches maximum LOVE (Level of Violence). Most of the time, when the Anomaly resets time, only Frisk and Flowey are fully aware of the resets while everyone else has only vague recollections but under certain conditions even Frisk and Flowey can be made to forget. The only ones who are immune are those who reach maximum LOVE, such as Chara and Asriel.
There are only a handful of beings in Undertale that are aware of the Anomaly's influence and none of them know all of the details. While the Anomaly has been mistaken as Chara and Frisk by Flowey and Chara respectively, it's unknown what Frisk knows and Sans isn't aware that the Anomaly is only a recent appearance and that it was Flowey who first caused the timeline anomalies.
The only beings capable of challenging the Anomaly directly are those who have reached maximum LOVE, such as Chara (who gained their LOVE through murder) and Flowey (who gained his LOVE by absorbing human souls). However, both are powerless against indirect influences as the Anomaly can edit the games files to alter the world as they see fit.
In a pacifist run, the Anomaly is a benevolent influence on Frisk who guides them through the underground, being helped by Chara who gives them advice on the best course of action. However, at the end of a pacifist run, it is possible to reset back to when Frisk fell into the underground, which by that point even Flowey no longer wants to do.
However, if the Anomaly had done a genocide run before, the ending will be made sour by Chara who shows that they can possess Frisk at any time due to the Anomaly having sold Frisk's soul, regardless of how many Pacifist Runs they done.
The influence of the Anomaly can be either benevolent or malevolent in this run. They can still be helping Frisk through the Underground, but not on a pacifist route due to mistakes on their part. However, if the Anomaly is experimenting at this point then they are trying to find out what would happen if they kill certain characters and spare other characters, under the justification that everyone can simply be brought back.
In a genocide run, the Anomaly is a villainous demiurge, playing an emotionally exhausting playthrough that calls them out on their actions every step of the way, is full of tedious level grinding, gives little pleasure in its fights due to almost all of those being incredibly easy while the two difficult boss fights are an exercise in frustration and permanently ruins any ending they can get for no other reason than that the Anomaly can and because it can, it must. At any point, the Anomaly can undo a genocide route by resetting unless they reach the point where Chara speaks to them.
After Flowey 's and Asgore's deaths, Chara greets the Anomaly, thanking the latter for guiding and teaching them the importance of power. Chara then gives the Anomaly the option to erase the world or preserve it. Both options lead to the world being destroyed, but saying to preserve it results in Chara betraying the player.
If the Anomaly returns to the Underground, it will be a Chaotic void. After ten minutes, Chara will speak to them again, knowing the Anomaly wants to go back and will call them out for thinking that they are above consequences. Chara will then offer to make a deal with the player, trading Frisk's soul in exchange for regaining their influence over the world. If the Anomaly refuses, Chara will say to stay there forever. Though if the player agrees, then Chara will uphold their end of the deal.
If the player does a second Genocide run, Chara will be disgusted by the Anomaly's actions, describing the Anomaly as having a perverted sentimentality that they can't understand and viewing the Anomaly as a different and worse kind of depravity than their own. They will even suggest to the player that a different route would be better suited should they choose to recreate the world again. Nonetheless, Chara will then give the same options as the first Genocide run, erasing the world either way. However, they will not prevent the Anomaly from resetting their second genocide run.
In Undertale's successor Deltarune, the Anomaly takes control of the human Kris. However, before all this, an unknown Voice, speculated to be the infamous Dr. W.D. Gaster, speaks to the Anomaly and tries to create a vessel with them. After it's created, however, someone (either the Voice or a new entity) discards it, declaring that the Anomaly's choices don't matter.
The entity's words were true, as the relationship between the player and the game is reversed: now your choices don't matter. This time, the player's choices are powerless and the game is successful at limiting their actions regardless of their path. Attempting a Genocide Route in Deltarune is insignificant as the enemies will automatically flee when defeated and no one can be killed. Meanwhile, attempting a Pacifist Route will fail, as you end up meeting a truly evil villain within the game's world itself whom cannot be reasoned with. The Anomaly's philosophy of the Genocide Route has been integrated into the characters within Deltarune: Susie fights enemies on instinct, the King is irredeemably cruel towards the other characters and refuses to listen to reason, and Jevil believes that he can get away with his actions because he knows that the world of Deltarune is just a video game where he can do whatever he wants without consequences.
At the end of Deltarune's first chapter, No matter what route the Anomaly took, Kris gets out of bed and tears their SOUL out of their body and throws it into a cage, before making an evil look on their face while brandishing a knife. Meanwhile, the Anomaly is able to move the SOUL around in the cage, suggesting that they were in control of Kris before they threw the SOUL away. Now, Kris has been possessed by a greater, pre-existing evil beyond the Anomaly's control. It is currently unknown who or what has possessed Kris, or what it's intentions are, but it's often speculated to be Chara.
The rest of Deltarune will be added when the other chapters are released.