Hidden Homunculus

The lone-wolf PC at your table is secretly an artificial being. Which is why they have no friends or attachments, because they are a facsimile of a conscious social being. But they imitate being human (or elf, or whatever) well enough that even the player running the character doesn’t know. Their status as an artificial being is revealed after a near-death experience. The medic patching you all up indicates (to other members of the team only!) that the PC is missing some rather important organs.