Mungo Treacle is a cheerful green goblin with long pointed ears that tilt forward when he listens. His round cheeks crease into deep folds when he laughs, usually squeezing his eyes shut, and there is almost always a pale dab of flour on one cheek. His broad, work-roughened hands are dusted white, with dough caught in the creases and small burns from years at the oven. He moves with a baker's practiced economy, shuffling quickly between counter and oven, then rocking back on his heels when a customer makes him laugh. His breath carries a faint warm, yeasty wheeze after he has hauled a tray from the oven, a sound that turns to a sharp, held-in puff when he is startled. For a man who spends his life around hot ovens, he is oddly fond of cold rain and will stand in it just to smell the wet earth.

Mungo is openhearted and quick to laugh, yet has a stubborn streak that appears whenever someone is ignored or belittled. He is patient with children and anxious customers, but brisk with anyone who wastes food. When relaxed, he fills silences with jokes and samples from the day's baking. Under pressure, the jokes stop, his breath catches, and he becomes plainspoken and unexpectedly firm.

Mungo was raised in a family of goblin charcoal burners who moved between villages with the seasons. As a boy, he was blamed for a disastrous oven fire at the old mill bakehouse. He had tried to warn the adults that the chimney was clogged, but they laughed at his nervous wheezing and sent him to fetch more fuel. When the roof caught, Mungo pulled the youngest human apprentice, Bess, out through a flour hatch. The fire left him with a lasting rasp after heavy exertion and a resolve to make a place where no one was dismissed as too small or strange to be heard. He apprenticed under a traveling baker, saved for years, and eventually opened his own shop in the village of Bristleford, a name locals say was worn down from an older word for a marsh crossing. Bess, now the village reeve, remains his closest friend, though she worries that Mungo gives away too much bread to people who cannot pay. His younger brother Kett still works the charcoal tracks and keeps urging him to return to the family trade. Lately, Mungo's flour deliveries have arrived short, and someone has been leaving damp ash in his oven vents.