Martha is a stout, immovable presence behind her bar. Her skin is permanently stained a matte charcoal gray from decades in the lower tunnels, making her sapphire-blue eyes pop with a startling, gem-like intensity. Her iron-gray hair is pulled back into thick, utilitarian braids, often containing faint, harmless embers that refuse to go out. The most striking feature is her right arm, a masterwork of obsidian plating and polished brass gears that whirs softly when she moves. The arm frequently vents tiny puffs of steam from the wrist. Despite the soot, she smells surprisingly of cinnamon and expensive tobacco.

Martha is the fierce mother-hen of the Blackstone mining district. She is intensely generous with the local workers, often extending 'forever credit' to those injured on the job, yet she is notoriously miserly and cold toward any patron wearing silk or carrying a High District seal. She speaks with a blunt honesty that borders on cruelty, but her actions always lean toward protection of the downtrodden. She harbors a quiet, simmering rage that only surfaces when the Blackstone nobility is mentioned.

Twenty years ago, Martha was the lead sapper for the Blackstone Mining Consort. During a deep-vein excavation, a pocket of volatile gas was detected by the High District overseers, but they ordered the blast anyway to stay on schedule. The resulting collapse took Martha's right arm and killed six of her kin. Using the hush-money meant to buy her silence, she instead purchased a dilapidated tavern and commissioned an eccentric gnome tinkerer to build her a new limb. Now, The Soot-Stained Mug serves as the unofficial headquarters for the Low-Tunnel Labor Union, and Martha is their undisputed voice.