Kadua tryblium is a scandent shrub with glabrous stems that can grow up to 5 meters long. The stems are few-branched, arcuate, and sometimes root where they come into contact with the ground. The leaves are simple and opposite, with oblong–elliptic to elliptic or narrowly elliptic blades that are slightly asymmetrical, measuring 13–22 cm long and 3–7.5 cm wide. The apex is long–acuminate or sometimes attenuate, while the base is cuneate or occasionally attenuate. The surfaces are glabrous, with the upper side green and the lower side pale green, often irregularly marked with purple or brownish-purple areas. The leaves are coriaceous with entire margins. The lateral veins are strongly ascending and slightly arcuate, with higher-order venation obscure and only a few evident veins. The petioles are 1.8–5.5 cm long, with a grooved upper surface due to stipular scars. The stipules are membranous and completely adnate to the petioles, forming a cup 25–40 mm high that collects water and debris. As the stem and leaves elongate, the stipules quickly become ripped along the petiole margins and are caducous by the second or third node. The stipule apex is abruptly acuminate and prolonged into a thinly fleshy mucro 7–9 mm long. The flowers are arranged in dense, axillary thyrsoid inflorescences, with peduncles ranging from 1–12 mm long and pedicels 1–2 mm long. The bracts are connate and ovate, measuring 6–10 mm long. The hypanthium is turbinate and 1.5–2 mm long. The flowers are insect-pollinated, heterostylous, and appear bisexual (perfect) or functionally pistillate. The calyx is 4–5 lobed, with ovate lobes that are 2–3 mm long and have fimbriolate margins. The corolla is yellowish-green, with a reddish-purple-tinged tube, fleshy, and funnelform. It is slightly dimorphic, with 4–5 lobes. In short-styled flowers, the corolla bulges at the throat due to underlying anthers, whereas in long-styled flowers, it scarcely bulges. The tube is 6.5–9 mm long, and the limb expands abruptly from the tube in bud, strongly quadrangular. The lobes are broadly ovate, 2.5–3 mm long, with a minute appendage at the apex. The stamens are as many and alternate with the corolla lobes, inserted on the corolla lobe. The anthers are fertile in short-styled flowers, while in long-styled flowers, they are sterile. The anthers are dithecal and open by longitudinal slits. The ovary is inferior or partly so, with 2(-4) locules, and contains few to numerous ovules on fleshy placentas near the middle of the septum. The style corresponds to the number of carpels and is glabrous and 2-lobed. The lobes are firmly appressed together in short-styled flowers, while in long-styled flowers, they are exserted and spreading. The stigma is bilobed or subcapitate, either included or exserted. The fruit is drupaceous and subglobose, measuring 4–5 mm in diameter when dry, and is indehiscent. The seeds are dark brown, irregularly shaped, and minutely papillose, with well-developed oily endosperm, though sometimes the endosperm may be scanty or absent. It occurs on steep slopes of the windward (eastern) and, less commonly, the leeward side of Mount Kāhili, along Wahiawa Stream, and in one population just below the upper rim of Kalalau Valley on Kauaʻi.