Kadua axillaris grows as shrubs or small trees and occasionally as lianas, reaching heights of 2–6 meters. It features simple, opposite leaves that are usually evenly spaced along the stem. These leaves range from oblanceolate to elliptic or obovate, measuring 6.4–16 cm long and 2.6–6.5 cm in width. The leaf apex varies from short-acuminate to acute, sometimes obtuse or rounded, while the base may be obtuse, cuneate, or attenuate. The upper surfaces of the leaves are glabrous, whereas the lower surfaces can be sparsely short-hirtellous, especially along the principal veins, or entirely glabrous. The leaves are membranous to chartaceous with entire margins and inconspicuously arcuate lateral veins. The flowers are presented in paniculate inflorescences that appear axillary or on very short, leafless, lateral branches, and sometimes terminal. These inflorescences measure 2–6 cm long and are either short-hirtellous or glabrous. The flowers are insect-pollinated and can be either bisexual (perfect) or functionally unisexual, often displaying heterostyly. The calyx lobes are erect or slightly spreading, narrowly deltate to linear-deltate, and are 0.5–0.6 mm long. The corolla is greenish white, with the tube occasionally showing a reddish tinge or purple, forming a salverform shape, with the tube measuring 4–6.5 mm in length. The fruit is drupaceous, dark blue or purplish-black, and measures 4–6 mm in diameter. The persistent calyx lobes remain erect to somewhat spreading. The seeds vary in shape (hat or fan-shaped and laterally cuneate and compressed or ovoid to ellipsoid with conspicuous bubble-shaped bodies included in the cells) and may have a well-developed oily endosperm. However, the endosperm can occasionally be scanty or absent. It inhabits mesic to wet forests between 400-1830 meters.