Kadua formosa is a subshrub that can grow up to 1 meter tall, characterized by a prostrate main stem that is hollow and somewhat quadrangular or 4-costate, with a glabrous surface. The leaves are simple and opposite. The leaf blades are lanceolate to elliptic or oblong, measuring 5.5–14 cm in length and (1.6–)2.3–4.5 cm in width. The apex of the leaves is acuminate, and the base is cuneate. The leaves are coriaceous with entire margins, and the lateral veins are strongly ascending. The higher-order venation on the lower surface is conspicuously reticulate. Petioles are short, ranging from 0.2–0.4 cm in length. Stipules are broadly deltate, measuring 3.5–4.5 mm long, adnate to petioles, and forming a persistent sheath with a long acuminate apex. The flowers occur in dense corymbose inflorescences that are subtended by two cordate bracts, often with a lateral inflorescence in each. The central flower of the ultimate cyme is sessile or on a pedicel shorter than the two lateral cymes, with lateral pedicels approximately 1–2 mm long. The hypanthium is cylindrical-turbinate, about 2 mm long. Flowers are insect-pollinated, bisexual (perfect), sometimes functionally unisexual, and often heterostylous. Calyx lobes are bluntly deltate, small, and do not enlarge in fruit. The corolla is white, fleshy, and salverform with a tube 20–26 mm long, and the lobes are approximately 6–7 mm long. A nectary disc is present. Stamens are alternate with the corolla lobes and inserted somewhat below the corolla throat; anthers are sessile or on short filaments and dithecal, opening by longitudinal slits. The ovary is inferior or partly so, 2(-4)-locular with few to numerous ovules on fleshy placentas near the middle of the septum. The style is as many as carpels, glabrous, with lobes flattened, terminal, and slender; the stigma is bilobed or subcapitate, included or exserted. The fruits are ellipsoid–cylindrical capsules, measuring 9–12 mm long and 3–3.5 mm in diameter. They dehisce loculicidally across the disk and then septicidally with a heavily sclerified endocarp and a depressed–conical disk. Seeds are dark brown to nearly black, peltate, angled, and contain a well-developed oily endosperm, although the endosperm is occasionally scanty or absent. It is known only from wet cliffs on Maui.