Kadua affinis is a shrub, liana, or small tree reaching heights up to 5 meters. The leaves are simple, opposite, and can be obovate, oblanceolate to elliptic, oblong, oblong-lanceolate, or sometimes suborbicular, measuring 2–18(–23) cm long and 0.8–8 cm in width. The leaf apex may be rounded, obtuse, acute, or short-acuminate, with a base ranging from cordate to truncate, cuneate, or attenuate. The upper surfaces of the leaves are typically glabrous, while the lower surfaces may be glabrous or short-hirtellous, particularly along the veins. The leaves are described as membranous to chartaceous or coriaceous, with entire margins and moderately to strongly arcuate lateral veins. It produces flowers arranged in terminal, open to very open or congested, paniculate inflorescences measuring 3–25 cm long. These inflorescences can be short-hispidulous or glabrous, with peduncles typically 4–35 mm long. The flowers themselves are bisexual (perfect) or functionally staminate and are pollinated by insects. They feature a calyx with 4-5 lobes and a corolla that is purple or green on the tube, with yellowish-green or sometimes purple-tinged lobes. The corolla is salverform, glabrous, or short-hirtellous externally, with a 4–15 mm long tube. It produces drupaceous fruits that are dark blue to purplish-black, measuring 4–12 mm in diameter. The fruits are indehiscent, with persistent calyx lobes that are erect to slightly spreading. It inhabits mesic to wet forests, occasionally found in bogs or mesic shrublands between 260-2040 meters.