Kadua flynnii is a compact, many-branched shrub that grows (20–)30–75(–100) centimeters tall. The stems are glabrous, with young stems being 4-costate and older stems 2-costate, with bark that weakly exfoliates. The leaves are simple and opposite, usually closely spaced and overlapping, gradually reducing in size distally along the stem. The blades are lanceolate to ovate, rarely linear–lanceolate, measuring 2–6 cm long and 0.5–1.5(–2.2) cm wide. The apex is long acuminate to long attenuate, often falcate. The base is cuneate, rounded, or rarely truncate. The blades are coriaceous, with flat to occasionally weakly revolute margins. Lateral veins are present in 2–4 pairs on each side, strongly ascending and prominent on both surfaces. The tertiary venation is prominently reticulate on both surfaces, often with a reddish-purple tinge. The petioles measure 0.2–0.8 cm long, are glabrous, and often exhibit a reddish-purple tinge. The stipules are deltate, glabrous, and aristate, with a stiff awn 1.5–5 mm long, often also tinged reddish-purple. The flowers occur in dense, 7–15-flowered corymbiform inflorescences, subtended by a pair of ovate bracts, with smaller bracts subtending the inflorescence branches and pedicels. The flowers are unisexual, insect-pollinated, dimorphic, and dioecious. The calyx is 4- to 5-lobed, with lobes oblong or ovate to narrowly triangular–subulate, 2–6 × 0.5–2 mm, subequal, rigid, and venose. The corolla is salverform, with valvate lobes that are conspicuously inflexed in bud, and the apex depressed in bud. The tube is greenish-white, rarely with a pink tinge, and 4- to 5-lobed, with pure or greenish-white lobes at anthesis. A nectary disc is present. The corolla tube measures 11–14 mm long and 2–2.5 mm in diameter in staminate flowers, while the lobes are elliptic to oblong–lanceolate, 7.5–8 × 1.5–2 mm. The corolla tube is shorter in pistillate flowers, measuring 7–8 mm long and 1.2–1.5 mm in diameter, with lobes measuring 2.5–4 × 0.8–1.5 mm. The stamens are as many as and alternate with the corolla lobes. In staminate flowers, the stamens are inserted 2–4 mm below the apex of the tube, with anthers measuring 1.8–2.8 mm long and filaments 0.2–0.5 mm long. In pistillate flowers, the stamens are inserted 1–3 mm below the apex of the tube, with anthers 0.8–1.5 mm long and filaments 0.3–0.5 mm long. Pollen does not develop in pistillate flowers. The ovary in pistillate flowers is ⅔ (rarely to ½) inferior. The style measures 6–8 mm long, 0.8–1.1 times the length of the tube, and is woolly-villous in the lower one-third to one-half. The stigma lobes are ovate-elliptic, 1–1.5 mm long, and free. In staminate flowers, ovules are present at anthesis but never develop. The style measures 5–8 mm long, 0.5–0.7 times the length of the tube, and is woolly-villous in the lower one-third to one-half. The stigma lobes are poorly developed and coherent. The fruit consists of subglobose capsules measuring 5–6 mm long and 5–6.5 mm in diameter, dehiscing loculicidally across the disk and later weakly separating septicidally. The calyx lobes are accrescent and erect, with the disk raised, measuring 1.5–2 mm long. The ovary is ⅔ (rarely to ½) inferior. The seeds are irregularly ellipsoid–angulate or subglobose-angulate, measuring approximately 0.6–0.9 mm long, with a dark brown or black papillose seed coat. The endosperm is well-developed and oily, though it is occasionally scanty or absent. It is usually restricted to north- and northeast-facing cliffs and steep, narrow ridge crests and outcrops, though it is less commonly found on steep rocky slopes and the upper portions of basalt cliffs. It generally occurs in windswept areas with small populations of 30 to 50, in relictual lowlands, and in montane-diverse mesic forests and shrublands between 350-1,100 meters.