Aku

In default form, He appears as a being similar to Sans although he has noticeable differences such as, being taller, a deformed face and dental structure, and his disgusting "organs".

In his Human Disguise, he appears as A tall man wearing a full black suit. His face is covered in tumors, he lacks eyes, has an extremely big set of teeth that cover a very extensive chunk of his head and wears a hat that resembles a fedora.

He likes being morbidly creepy to those who have the bad luck to encounter him, causing several mental illnesses to them, and visiting those who fell victim to his trap on their last couple of minutes before they pass away. He dislikes people escaping his grasp and almost every other EXE.

Once you encounter him. He'll make your life a living hell; he's going to talk trash and say several disgusting things about you. He'll also inform you that he can feel that you're getting uncomfortable with sentences like:

"i can hear you crying, keep going."

He's going to miss a lot of his attacks on purpose to make you stay more time inside his battle and consequently break your mind. He finds a lot of pleasure in his victims' suffering. All of this is with the purpose of getting his victims to develop mental illnesses like depression, schizophrenia, dementia, etc. In the final moments of a victim's life, either on their deathbed or just mere seconds before they decide to end their own life themselves. He'll appear and tell and show them everything they missed in their lives, just because of him randomly deciding to appear on their playthrough. Laughing in a deep, low voice as they draw their final breath.

Aku is an anomaly present in any Undertale copy ever since its release, although he only became active about a year after said release. He'll appear at any given point in the game at random and force you into his encounter. (Note: The anomaly will NOT replace Sans in-game.)

Every time you attack him, a shield of sorts pops up that absorbs all the damage dealt.

His "organs", or at least, that's how HE calls his malformed tumors, with teeth and eyes in place of his inner organs, will extend out of their place and attempt to bite you.

Since Aku is made out of individual pixels. He's able to change his own body and theoretically transform into other characters (He will always be 96 pixels tall).

Aku can appear in real life, only visible to those who he allows to see him.

Used mainly to set up his massively high stats and trigger his encounter.

The shield always stays in front of his center of mass, therefore attacks from the back or sides can affect him, which is his first weakness.

His second weakness is Cutting his limbs or organs when he extends them far enough that they get out of the shield's range can also work.

He lacks his extremely high durability in real-life, meaning you can kill him when he gets out of your screen (though that would be extremely difficult).

Aku isn't linked to any piece of code in the game. He only resides inside it. For this very same reason. He can only go after one victim at the same time. For this very reason. He exists in every Undertale mod (E.g. Undertale translated to Spanish, Undertale: Bits and Pieces, etc.) There is an alternative version of Aku called "Smiley". Aku can speak any language regardless of where it's from. If you somehow manage to not kill yourself or end up in a mental asylum, Aku won't just give up. He will go after you on every Undertale copy you play. No matter when or how, he will always appear. Not playing Undertale never again won't save you either; he will appear in real life and torment you personally. Trying to close the game will result in the game becoming unresponsive, though Aku will persist to talk directly to you. If you could, in theory. Break Aku's shield; the result wouldn't be so impressive. You could get a single hit in that would deal little to no damage even with hacked stats, and then the shield would regenerate. Though it is literally impossible in practice since the shield is unbreakable. Aku can't stop smiling. It doesn't matter if he's sad, happy or mad. He will NOT stop smiling.