Role in the story
Lunka is Coen's sister and the reason everything in The Blood of Dawnwalker happens. When she contracted the Black Plague, the family was out of options. Brencis's vampires intervened, soldiers were about to kill Lunka when the vampires arrived, killed the soldiers, and cured her with vampire blood.
The cure worked. Lunka survived. But what looked like salvation immediately became leverage. Brencis used the debt to hold Coen's entire family hostage, trapping them in an arrangement they couldn't escape. The exact terms (what Brencis wants from the family and why) remain part of the game's central mystery.
Effects of the vampire blood
Lunka was cured by drinking vampire blood, which is how the plague cure works for all humans in Vale Sangora. But creative director Mateusz Tomaszkiewicz has suggested that the vampire blood may have had additional effects on Lunka specifically. She may be aging slower than normal humans as a result of the cure.
Tomaszkiewicz hinted this could carry implications for potential sequels, suggesting Lunka's story extends beyond the events of this game. Whether the blood changed her in other ways (giving her abilities, altering her perception, or connecting her to the vampire hierarchy in some fashion) has not been revealed.
As a character
The developers have described Lunka as "a cryptic figure important to the story's exploration of survival, morality, and power." That phrasing goes well beyond a simple rescue motivation. It suggests she has her own arc, her own agency, and possibly her own secrets. She is not just waiting to be saved.
She is one of Coen's four siblings. Rescuing her and the rest of the family is the objective of Coen's 30-day mission. Beyond that basic fact, the developers have kept her story deliberately obscured, which in itself suggests there's something worth hiding.