Mako, Yellow and Cyan

Main story:

(this is intended to be an isolated, complete story. think arcs 2 onwards are the series being renewed for another season lmao)

in this universe crewmates are not humans, they're their own species of space-fairing biomechanical robots who self-propagate thru bioengineering. it is simply how their culture is. they require their "suits" to stay alive, maybe they were originally made by humans or some other alien species and then broke off into their own thing. anyway, the impostors are another alien species that targets them. the alien species is most known for targeting ships in transit that have either broken down or are forcefully shut down by the impostor themselves. when this happens it's called, literally an "impostor scenario."

Yellow, has been training to be an agent of a military agency that specializes in exterminating impostors before they can seed in bases on planets. this military base, notably, sends teams of flamethrower-wielding agents with handheld bioscanners to halt impostor scenarios in their tracks, scan the crewmates and destroy the monsters. The impostors are constantly evolving which means these bioscanners must be updated often, they are scarce and only given to members of this agency. on his way to the main base of this group, the ship shuts down and an impostor scenario occurs. he vaguely knew Mako before this, so they try to stick together. however, Mako is, Of Course, one of the impostors on the ship, along with Red. Yellow gets targetted by the other crewmates for their odd behavior. unfortunately Mako has gotten instantly attached to Yellow, and clears them. however, Yellow has started to figure out that Red is an impostor.

Red confronts Mako about defending the crewmate, and Mako is able to get him off his back, but Red insists on killing Yellow to prevent this from going on any longer and also to prevent him from figuring out what they are. Yellow gets cornered by Red, who is about to kill them when Mako shows up, and transforms, attacking Red in defense of Yellow, revealing himself as the second impostor.

Yellow manages to escape and call an emergency meeting, where they call out Red, and get him ejected. the rest of the crew know there is a second impostor as well, but Yellow defends Mako and clears him too, despite seeing him morph. unfortunately, the ship, when it is fully repaired, as well as the military base, have bioscanners that could be used to immediately reveal and kill Mako, and the rest of the crew are slowly closing in on ejecting Yellow as well. carefully, and reluctantly, Yellow and Mako kill off the rest of the crew.

at the end of everything, when everyone is dead, Yellow is able to fix the ship, and in order to preserve Mako's life, they have to separate, and Mako is sent back into space via an escape pod on the ship. Yellow arrives at the base of the agency, suspiciously alone, but after being cleared by the base bioscanner, is accepted into their ranks and praised for somehow managing to solo survive an impostor scenario.

he also has to live with the guilt of being directly responsible for the deaths of 7 innocent people but y'know it's whatever.

Arc 2:

Yellow has been a praised member of this agency for a long time, has completed his training and has, with the help of his team, killed dozens of impostors and curbed many would-be impostor scenarios.

notably, Yellow has become very reserved (even more than he was before lmao) and doesn't interact with his peers much, despite being in a much more friendly environment than his past. despite this, he eventually becomes fairly good friends with the head scientist and one of his teammates, Cyan, due to their shared natural intelligence and, ironically, social anxiety. Cyan is in the process of making a new, mass producible prototype for the handheld bioscanners that can be updated wirelessly, which could be provided across their whole species and allow them to completely wipe out the impostors all over the local galaxy. 

Cyan is particularly curious about Yellow arriving at the station alone, having been something that never happened before and he gets relatively uncomfortable whenever it is mentioned. However, one evening when they are chilling and talking in the rec room, Cyan asks Yellow if he'd ever been interested in anyone. She notices that his avoidance of the subject is very similar to how he behaves when asked about his arrival.

Eventually when they are on a mission, Yellow scans a black impostor who looks very similar to Mako and breaks down, refusing to kill them, putting the ship in danger when the rest of his team has to react to quickly put it down. When they're back at the base and alone, Cyan presses Yellow and he finally opens up, talking about Mako, and even seeming happy to do so, becoming even somewhat bubbly. Cyan asks what happened to him and Yellow goes quiet. After some more pressure, Yellow finally breaks and tells Cyan everything. Being extremely loyal to her cause, and her ultimate goal to literally exterminate the entire species, Cyan is  conflicted and almost even regrets pressuring Yellow to tell her in the first place, but to both of their surprise, she comforts him and promises to keep his secret.

One unfortunate day, the impostors mutate to completely evade the bioscanners, even those at the military base, and pressured by the fear of the encroaching technology, the impostors form a massive horde and invade it. The building is destroyed, and dozens of members are killed including a majority of the head scientists and leaders. Yellow is unable to find Cyan in the chaos, and the impostors are imitating other survivors making it impossible to trust anyone. So, equipped with a flamethrower, Yellow is left defending himself against an onslaught of monstrosities as he tries to fight his way through to the hangar at the far end of the base, where he may be able to escape on a shuttle before the horde reaches it.

In a hallway, Yellow is forced to cut the canister of his flamethrower open and light a wall of fire to cut off the horde from following him, and drops it so he might be able to run faster. Through the flames, however, he spots a familiar black crewmate looking around frantically. Their eyes meet, and Yellow calls out, but a vent cover falls behind Yellow as another impostor ambushes him, skewering him through the gut with a massive claw. Mako transforms and leaps through the flames, ripping the other impostor apart, and grabs Yellow, running for the hangar.

When he makes it to the hangar, Cyan is there with her latest prototype, updated personally, and scanning and ushering survivors into a shuttle. However, when Mako steps up to the door, holding Yellow's unconscious body, she presses the scanner to him and it alerts her that he's an impostor. He begs her to just take Yellow and leave, but she grabs him by the collar, and virtually throws him into the ship as the horde floods into the hangar. They're able to barely shut the doors and take off unscathed.

Inside, the survivors are recovering from their panic, as Cyan and Mako rush Yellow to the medbay. Mako sets Yellow on a bed and Cyan gets to work stitching him up and repairing his suit. As she does Mako watches in silence, and eventually speaks up, asking Cyan why she allowed him on the ship. She states that she knows who he is, and that if Yellow trusts him, then she trusts him too, but makes it abundantly clear that he won't be hurting a single person on the ship, and he'll be providing her with biosamples so she can keep her bioscanner updated. Mako agrees.

(will write more later but holy shit sorry this is so wordy)

Uses he/him pronouns.

The scenario where he met Yellow is his first time killing a ship. For an impostor, he is very young and inexperienced.

The crewmate whose body he took used to be friendly acquaintances with Yellow during their training a long time ago.

In personality, he is kind, but he's still an Impostor. His intelligence and sentience is inherited from the body he's using, otherwise the personality he has would be entirely overshadowed by his species' primal instinct to kill. He can be a bit dumb, and makes decisions without giving them much thought. The decisions he does make, he tends to based more on emotion. (aka, betraying his entire species because he got a little crush on a crewmate he'd only known for like an hour or two)

Uses he/they pronouns.

Socially anxious, and notably neurodivergent. Often the target of aggression from the people around them despite being equally, if not more capable. Comes from a rough home life and has learned to keep his head down. Yellow has a learned belief that the universe is actively hostile to them specifically due to constant misfortune in the life that they have worked very hard to build.

Yellow is quick to open up to Mako about their problems due to him being the only person they thought they knew on the ship.

Despite falling into helping Mako so easily, Yellow is actually incredibly smart, and thoughtful about his decisions. Though his choice may have been based on emotion, he has considered the logic, and simply chosen to ignore it. This doesn't mean Yellow isn't heavily affected and guilt-ridden about what he's done, but it's a consequence he accepted willingly.

Uses she/her pronouns

Cyan is another agent of the organization that Yellow works for. She is a head scientist working on handheld bioscanners that are used to directly combat impostor scenarios on targeted ships. Needless to say she is super smart and also very aware of the intentions of people around her.

Cyan met Yellow after he arrived at the base, and became Yellow's best friend. She's the first, and only person he told about what actually happened on the ship he arrived on. 

She's incredibly loyal to her cause, but loyal to her friend as well, and keeps his secret for his sake.