Spoilers ahead
Spoiler note: Thorne's full role in the story is spoiler-sensitive.
Thorne is the principal antagonist of Mina the Hollower — a bat who served as the former head of Baron Lionel’s guard. He betrayed the Baron and, alongside his shock troopers, destroyed the Spark Generators that powered Tenebrous Isle, leaving the island dark and overrun by monsters. He’s the figure standing in Mina’s way.
He also doubles as the game’s first major boss. For attack tells, counters and the best weapon, see How to Beat Thorne — and note that he flees on the brink of death rather than dying in that first fight.
True role (spoiler)
Multiple reviews indicate the story deliberately subverts the “obvious villain.” Kotaku references “a ham-handed way to signal the true antagonist” and a major end-game choice “that determines the fate of just about everything,” so Thorne’s complete role and the identity of the true antagonist are spoiler-gated. See the Story & Ending page for more.
FAQ
Who is Thorne?
A bat and the former head of Baron Lionel's guard. He betrayed the Baron and, alongside his shock troopers, destroyed the Spark Generators — making him the antagonist standing in Mina's way.
Is Thorne the final boss?
No. Thorne is the first major boss, not the final one. He flees your first encounter rather than dying.
Why does Thorne betray Lionel?
His full motivation is spoiler-sensitive. Reviews note the story subverts the "obvious villain" setup, so Thorne's complete role is best left to the story page.