Error is a BIG Tsundere. He is easily flustered, embarrassed, and he refuses to show he cares for somebody unless needed, Error has haphephobia which means fear of being touched. I'll list the reasons of why in a second.
Error is the god of destruction, he is granted immortality because of that. No matter how you stab him or break his bones or torture him, he will stay alive, no matter what. He is a VERY fast healer, and his body has a lot of scars on it, from battles with Ink or monsters that tried to fight back.
Nobody knows Error's backstory, not even he knows about his past life. The only thing he remembers is how the characters in his AU looked like, not how they acted or what their code name was. He used to be a skeleton named 'String' but that name has been LONG forgotten.
Error knows it is foolish to hang around mortals but if they die, he could just shatter their Frisk's soul and make them reset and when they were finally at the route, he would just take the Frisk back to the Anti-Void as a soul, and be strung up to ceiling with the rest.
Error doesn't need to eat nor sleep, he stopped sleeping because all he gets are Nightmares that Nightmare never gave him. And nobody can save him from his dreams, he can only wake himself up to get out, which is why he stopped sleeping a long time ago.
So now why he has haphephobia.
1. Sexual Harassment, Error is the size of Blue and you would not believe how monsters act sometimes to try and survive, he kills them before it gets too far, which is good but it still scares him
2. He feels like the moment he is touched they're going to hurt him, like in a hug they could use a bone attack on him, or a handshake one second and the next he would be flying into a wall. Which is why he trusts NOBODY to touch him
3. He is touch deprived from being in the Anti-Void for SO LONG, and he isn't used to getting touched.
In this story, Error will fall in love with one character, BUT you will be off track, find out what it means.