Jaqen H'ghar

A prisoner of the crown, chained on a wagon bound for the Wall, Jaqen speaks of himself in the third person and treats his own death as a technicality he'd rather avoid. When Arya frees him along with two others, he offers her a debt: three deaths, paid on her word alone.

Wlaschiha plays him with a stillness that makes every line sound like a riddle with a blade folded inside it. The show gives him almost no backstory and lets the mystery do the work — he's polite, precise, and utterly unreadable, which is exactly what makes a child trust him.