The “quirk”. Something that your character does that none of the others do. Addiction to hallucinogenic substances is an easy one. Of course the shadows crawl for me! Or, specifically artistic. Painting still lifes? They alway have flies because your muse is a rotting corpse that is just off canvas. Mismatched socks because your particular demon can’t follow you for some specific reason. Obsessive compulsive when opening doors to avoid walking into the wrong dimension, or letting something crossover. An object that you fidget with that keeps you invisible to the otherworld that you’re hyper-realistically aware of. Of course you’re a manic mess when it’s misplaced. And paranoid. Heirlooms. The whatever-it-is has been a part of your family for generations, and handed down from parent to child (oh! be the oldest or youngest, or only gender of your family). The responsibility falls to the eldest son, the curse is put upon the youngest child and that’s why your poor, tired mother had 17 children before you, or, the curse is dormant as long as a daughter is born each generation. I dunno, stuff like that?
“And I’d do it again.” Robinson. Probably.