The Kept Man of the Princess Knight Novel Volume 3 was written by Toru Shirogane and illustrated by Saki Mashima.

The Dungeon City of Gray Neighbor is shaken by a series of tremors that warn of an impending monster stampede. When there's no sign of Aegis after the dungeon is evacuated, Matthew descends into the dark passageways to rescue the princess knight. The curse that steals Matthew's strength when he's out of direct sunlight renders him useless against monsters, but he's Arwin's only line of defense against Release withdrawal and dungeon sickness. Even if they make it back to the surface, her recovery isn't guaranteed. But no matter how hopeless the situation looks, he won't abandon her. Matthew will do whatever it takes to save Arwin—whether it's facing off against the nefarious Sol Magni cult preachers or braving the monster-infested capital of Mactarode!