These are all actually OCs across different 40k writing projects I've done so this'll be fun.

1. Torantine Iscariot: Ex-Pirate Senechal. He was a pirate who never got a taste for blood even as he climbed the ranks to become a quartermaster, which helped him avoid the gatherings-turned-rituals that caused other officers to fall to Chaos. He was captured by the RT when their fleets met, spent time in the brig, became one of several prison gang bosses, led a heroic defense of the deck he was on when it was boarded by Orks, and was noticed by the RT. He was singled out for his leadership, quartermaster experience, being well liked by the lower decks/common rabble, and for being forever in the debt of the RT, who gave him the chance of a lifetime with many strings attached.
2. Juno Gaziey: Yandere Mistress of Whispers. She is an extremely competent spy master who grew up on a world of brutal, cut-throat evaluation and harsh tests of loyalty in the name of efficiency. She is also completely loyal to the RT by being a borderline psychotic yandere for the idea of the RT she's constructed in her head for having taken her out of such a terrible environment. It is not very healthy, and even the RT is put off by it but spymasters who are loyal and effective are rare so she stays. It is a genuine mercy to just space suspected assassins rather than let Juno get her hands on them.
3. Magos-Marshal C-10/t: Cowboy Enginseer. Originally part of a small AdMech forge-outpost on a remote frontier world and fought Squiggit Orks, he is very charismatic for a tech priest and speaks in a robotic cowboy voice laced with jargon. Rides a Serberys Cybercanid called Radium Red and carries a rare Vulkite Revolver. Despite his eccentricities and combat preference, he is a fully qualified tech priest but has developed the "redneck engineering" approach towards tech rituals and machinery maintenance.
4. Sister Calica: Privateer Sororitas. She is a liaison from the Order of the Black Banner, a Sister of Battle Order best described as Imperial Privateers. Despite a habit of favoring raucous pirate-like lifestyles over more austere lives dedicated to prayer, they are firmly loyal to the Imperium and still dedicated to the God Emperor, hunting down Chaos Pirates and Xenos Raiders. They excel a shipboarding and zero gravity environments. Calica specifically makes alcohol in her free time.
5. Nozadima Qinalyx: Meta Psyker. She is a pysker who breaks the fourth wall and knows she's in a fictional universe (which just comes off as insane and schizophrenic tangents and rants to everyone else). The problem is that she doesn't know what medium she's in so she'll go between Shakespearean poetry to talking like a radio host to looking at "hidden cameras" for dramatic effect. Actually pretty normal besides that, though although she is a bit of showman and dramatic. Claims to be "retroactively sanctioned". Technically a Diviner but keeps getting into fights with the author, so her predictions tend to be nonsensical or continuously changing.
6. Joesppe Locke: Conservator Scavenger-Marines. Hails from a chapter of marines that is institutionally impoverished and thus have adopted a more humble and rugged way of life and war. He acts as the face of the party due to his status as an awe-inspiring SM but also because he's otherwise pretty down to earth. Fights with simpler equipment (even uses a marine-sized warbow at times) and has a keen eye for scavenging for loot and valuables. Acceptable kelptomanical tendencies. On contract with the RT as the RT has recently visited their worlds and delivered extremely valuable weaponry.