Mirth is a very small fairy with dwarf ancestry, no taller than a wine bottle. Her porcelain-white skin has a faint, cool sheen, and her doll-round face is dominated by enormous golden eyes. Two long, messy braids of golden hair hang low behind her pointed ears, their ends threaded with tiny pieces of hammered brass. Four translucent dragonfly wings quiver behind her, veined like amber glass and flashing green when they catch the light. She has a button nose, a wide, cheeky smile, and a wiry, extremely slender frame. She moves in sudden darts and hovering pivots, as if gravity were a suggestion she finds mildly funny. Despite her delicate appearance, she keeps her black boots polished to a mirror shine and taps their pointed toes against stone with the impatient certainty of a veteran drillmaster.

Mirth is exuberant and teasing with strangers, but becomes businesslike and almost stern when anyone is in danger. She craves freedom and hates being treated as fragile, yet she quietly longs for the dependable routines of her old home. She can be generous with comfort and ruthless with criticism, especially when someone mistakes kindness for weakness.

Mirth was born Mirja in a dwarven mining settlement carved beneath the Siltbreak cliffs. As a child she was fascinated by the glowing cave moths that gathered at the ventilation shafts, and she learned to repair delicate bellows and lift mechanisms for the miners. During a collapse, she squeezed through a narrow air duct to guide trapped workers toward a cracked emergency vent. The vent opened into a buried fey spring. Mirth survived, but the spring remade her: she became tiny, grew wings, and found that old runes answered her voice. Her first flight carried her away from home, and she spent decades learning to wield the magic that had saved her family. She now lives between the cliff settlement and the nearby fey paths, acting as a guide, negotiator, and occasional monster-hunter. Her former mentor, the engineer Drenna, believes Mirth abandoned the settlement after the collapse; Mirth has never explained that she was carried away before she could return. The settlement's air system is failing again, and Mirth is secretly trying to repair it before anyone discovers how much of its original design she altered.