I haven't finished the whole novel yet, but from what I've gotten through, it feels like there are fewer key players than you'd expect for a space opera. Cass, the viewpoint character, is obviously central. She's the pilot of the derelict scout ship 'Star's Rest' and her internal monologue about loneliness and purpose is pretty much the engine of the first third of the book. The other one who immediately stands out is Aris, the enigmatic being she finds floating in a damaged life-pod. Aris isn't human, and their communication starts off as pure gesture and intuition, which creates this interesting dynamic where Cass is trying to solve a puzzle just to have a conversation.
Then there's the voice on the comms, the one from the lost colony ship 'Elysian Drift'. That's Captain Renn, and even though he's just a voice for a long time, his decisions and the slow reveal of what happened to his ship drive a lot of the plot's mystery. The relationship between Cass and the ship's AI, which she nicknames 'Box', is also weirdly key. Box isn't sentient in the usual way, more like a echo of previous crews, but its fragmented data logs become crucial for understanding where Aris came from. So I'd say the core is this quartet: Cass, Aris, the ghost of Renn, and the ship itself.