Midnight Sans is originally from the AU Midnighttale. But after Cross Sans came to his Au seeking to kill him, only to be saved by Ink Sans, he asked Ink if he was able to come with him and help with keeping all the other AU's safe from Cross.
Midnight is a real go getter who is ready for anything that lies around the next corner, when if its a hostile, insane puppet collector to a fuzzy bunny who's lost and needs help getting home. He will literally stop and aid any monster, human or thing needing help. Midnight is super empathetic and doesn't like it when others get hurt even ones he doesn't know. So say if a werewolf was having a bad depressed day that just seemed to be getting worse, Midnight would go over to that werewolf and help him/her feel better. He is very hyperactive unable to sit still or stand still, no matter what he has to be moving even just a little. He is known to talk to himself sometime only because he thinks out loud and not in his head, some say he is a little insane and that half is true.
Midnight wears a dark purple hoody with magenta stripes on his arms, back and his fur on his hood is also magenta, for his shorts he wears indigo Nicke shorts with light blue stripes and blue and magenta converses. His hood is always up over his head he never removes it from there, its as if he's hiding something. His left eye is always closed, while his right one remains open with a magenta snake slit instead of the regular pupil and one of his teeth is a sharpened fang.
Midnight's past is one he wishes he could forget about. When he was only nine and a half he first used his magic to fend of some bullies beating up his younger brother Papyrus. His left eye that had been at the time normal had suddenly transformed into a rimed design with black in the middle then white, blue and yellow on other rims surrounding it. Midnight thrashed the bullies with such force and anger he began to lose himself, falling deeper into his insanity the more he used his magic. Papyrus tried stopping his brother but was thrown back into a tree, Midnight hadn't seemed to care that he had just harmed someone he loved so he then aimed a Gaster Blaster at Papyrus and shot. In the blink of an eye Papyrus was gone. The ashes of the tree that had once stood there were slowly drifting towards the snow when Midnight noticed his father was standing behind him with Papyrus in his arms. Shock and fear filling his watery eyes while Gaster's stayed cold and dead. It made Midnight angry to see that Gaster had saved Papyrus. So angry that he impulsed a huge amount of magic that it drained the life from the trees around them. Gaster placed the little shivering skeleton down on the ground while he himself fought Midnight. Quickly beating him with a full body Gaster Blaster. Not to long after that the royal guardsmen took Midnight away to a secret facility in Hotlands, where they would take out versus experiments on him. Gaster was the one who did these experiments because if he didn't they would forcibly take Papyrus away from him and rip both of the two apart and take their souls. Midnight under took many experiments, was tortured and kept in a cell with no windows doors or anything. When I was saying before that some people think Midnight is a bit insane, well he went insane inside the cell. And thats where he got the habit of talking to himself. After five years of being locked away Midnight was finally allowed back out into the undergrounds as King Asgore took the throne. Monsters all over the underground remembered Midnight and shunned him for his eye, that had never gone away, so he started only looking through one eye while keeping his rim one closed.
Midnight can (like every other Sans AU) summon a Gaster Blaster, glowing bones and weapons of his own kind like his giant bone battle axe and bone hand-blades. He can also send victims off into sleeps for long periods of time and sometimes if he's lost himself . . . for eternity.
His weaknesses are living through the horrible moments being experimented on again and again. His rim eye, since it possesses him into a rampage of insanity. Illusions, his kindness and empathy.