Matthew

Matthew, the Security Manager at MIRA HQ, was one of 12 people sent on the Skeld for a mission to a distant planet known as Polus. He, alongside the others, had to learn to get along while there, as they would not be seeing anybody else on the way. Stuff would become a disaster, however, when someone on the ship ends up attacking and killing the other Crewmates.

Matthew was called to the mission to be the Security guy on the Skeld, he was supposed to make sure everyone else in the crew was safe. He brought his daughter along on the trip because, otherwise, he wouldn't have seen her for months. Not that it would have mattered, anyway, for he ended up becoming the first victim of the Impostor. His body was found in the Communications Room, oddly not the Security Room he was usually found, none of the others had any idea who could have done it so they just left it there. The ghost of the Security Manager remained on the ship, doing several tasks idly, his presence discovered by his best friend when they saw his tablet while he was doing a Data Download. Though they could not physically see him, his daughter was reassured by the fact that he was still around them. At one point, he attempted to show who the Impostor was via the wires, connecting one wire that matched the colour of the person who killed him, but it wasn't figured out.
Near the end of the journey, nobody is really sure how, Matthew and the other victims were found perfectly alive again. They didn't know how it happened, or why they were back, but they were glad to be there for the trip to Polus.

At first, upon landing on the arctic planet, everything went smoothly. The person who had been killing on the Skeld had just stopped doing so, seeming to become a regular Crewmate again, everything went fine for their first few days or so. About a week in, one of their fellow Crewmates was found dead in the Medbay room, the resident Scientist had no clue where it came from, Matthew never saw anything suspicious on the cameras. Nobody was voted out, everything remained as it had for a few more days. The calm broke when Matthew discovered a body, calling an alarm, gathering everyone up. As much as he argued his own case, the others simply did not believe his innocence, and he was tragically thrown out and into the lava below. He didn't even try to fight back against it.
As with their presence on the Skeld before, when it came time for the journey to return home, the people who lost their lives on Polus ended up finding themselves alive again. Even Matthew, though he was left bitter by his wrongful ejection.

The crew soon returned back to Earth, where they could finally see their families again. Matthew parted ways with his daughter as he returned to his job at MIRA HQ, sitting in Communications and watching the Door Logs for suspicious activity. This was where he found out about how few people actually remembered that he existed. The main person working at MIRA HQ who spoke to him was their Scientist, the same person who had been on the journey to Polus with him. They knew each other well. Matthew would vent his feelings to her each time they spoke, she saw how bitter he was becoming, and eventually he volunteered to be a test subject for whatever experiment she was doing. At least it would make him feel important.
Matthew soon got his chance to take part in the experiment, fully accepting what he was putting himself into, stepping up to the Scientist and letting her do her work. Nothing was different for a day or so, that's what Matthew thought, at least until he suddenly gained the urge to harm somebody. His first victim was an unsuspecting worker. He found other people he knew from the Polus journey, and immediately a feeling of regret starts to grow in him. But not enough to give him any sympathy. Under the guise of just doing his regular job, MIRA HQ's Security Manager became a threat who was slowly eliminating everyone else around him.

It did not take too long for people to figure out who exactly was doing it. They had to make background plans to stop him, it was hard given how he knew the building better than anybody else working there, but soon enough they finally worked something out. A group attempted to confront him, several members ended up badly injured, but they eventually managed to subdue him and get him contained. He didn't bother fighting back when they finally decided what to do with him.
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Matthew currently lives with no memory of his previous actions. He still gets the occasional urge to rip into things, and to destroy, but otherwise he remains pretty calm and harmless. He's noticed that several people are scared of him, for reasons he simply can't recall, he doesn't realise that they are deliberately keeping quiet about what happened because it's the only way to keep everyone safe. He doesn't fully remember any of his friends or family, now only knowing them by base descriptions from vague memories he might have had?

He appears to be a pretty sweet and kind person, very caring about others, putting them above himself whenever he can. He doesn't even know himself, or what he is, all he knows is that he is probably a human and he can help other people? Matthew is holding onto his remaining shreads of humanity very tightly, but sometimes he can't keep himself in check, at which point he'll switch from calm to violent, attacking and likely killing the nearest person to him at the time, though afterwards he retains no memory of what he just did. It's like two people fighting for control over his body, with the nicer side winning until he lets his guard down.  He has found it hard to remain a member of society when at any moment he could suddenly turn against people and make himself look even worse.