The Story of Senna
Senna spent years inside Thresh's lantern — a soul trapped in the collection of the Chain Warden, aware but imprisoned, sustained by the Black Mist's weird half-life rather than dying as most souls in the lantern do. She entered voluntarily, to protect Lucian from Thresh's hunger, and she survived by the particular toughness that had always defined her: she is not someone who stops working when the conditions are bad.
Inside the lantern, she learned things. She spent years in intimate contact with the Black Mist, studying its patterns, understanding its nature, developing a relationship with the darkness that gave her capabilities no other living person has. The Mist is in her now — you can see it, the black wisps that trail from her arm, the shadow that moves wrong relative to the light. She uses it. She is not afraid of it. She has too much history with it to be afraid.
She was eventually freed and reunited with Lucian, and their reunion was everything it should be and also the beginning of a new complication: she is changed, and he has been hunting for so long that he is changed, and they are trying to rebuild a partnership in bodies and minds that are not quite the ones they had before. She fights the Shadow Isles with the specific authority of someone who has been inside what they're fighting. She also fights it with the anger of someone who spent years imprisoned by it.