Mert the Miller is a miller locked in Stoneward Post prison. He is a rebel contact, and you can free him in the Save Mert the Miller quest.
Where he fits
Mert is one of the early-game NPCs around Stoneward Post rather than a main-story pillar: the prison is tied to the early Stoneward Post side content, where the jail captain Friedo holds the Prison Keychain and the merchant Yenno is a conspirator you can threaten to learn more. Freeing Mert is an early objective and a point of contact on the rebel side, not one of the main ending-gating routes — those are the vassal arcs plus the Lacra and Crake chains.
For the full cast, see the key characters compendium.