if we're talking just arknights, then...

Dr. Saros of Rim Billition, ex-Laterano Spec Ops and Babel / Rhodes Island's tactical advisor, is a kangaroo man

Dr. Sunless / Sunrise, runaway Perro therapist from Bolívar, is the Doctor OC of mine who did get amnesia / goes through most of AK's main story, has so much depression

Sourire, Sarkaz Operator who grew up in a humble hamlet in Victoria until he contracted Oripathy, wandered the world helping people however he could as a wandering swordsman before joining up with Rhodes Island during the Lungmen-Chernobog crisis.

Aberration, an Operator from Aegir who's home and nearly his entire family was lost to the Seaborn, is a bit of a hardass but is one of the most compassionate and caring people aboard Rhodes Island who had been caring for him since he was young and frail

i have a few more but these are my main arknights OCs! (don't get me started on the rest of them outside of arknights or we'll be here forever)