I say a custom generated character with a premade generic voice acting and vague backstory that can fit the narrative. Heck it can be as simple as one of the tadpoles that escaped a dead host buried itself in its new host while they slept.
Without multi-player please.
It can be an option to create them at the start, and then we just meet them.
Or the hireling method...or whatever works best for you Larian.