Rillaboom is a bipedal Pokémon resembling a gorilla. Its body consists mostly of shades of brown: light, tan brown on its forehead, chest, abs, and extremities; dark brown on its nose and triangular ears, as well as under its eyes; and medium, mud-like brown across the rest of its body, including its thick arms and legs. Its mouth, the tip of its nose, and its irises are orange, and its sclerae are white. Each of its upper arms have tan marks resembling bow-ties or butterflies, and it has two, diamond-shaped, tan marks running down its spine. The first, and larger, of the two marks begins around its shoulder blades, and the second, and smaller, mark begins below its rib cage. Rillaboom has a thick bush of green, leaf-like hair on its head, which cascades down its back; on its forehead, a single, vine-like hair, which closely resembles a music note, falls between its eyes and curls at the end, which sits to its nose's left side. On its hips on either side, it has green, belt-like adornments resembling four leaves in a row. Similar adornments with two leaf-like parts sit on its each of its shoulders near the clavicles; these adornments appear to be part of its hair when not attacking. On each of its wrists, Rillaboom has cuffs that resemble leafy crowns, with the peaks of the crown facing its hands.
Rillaboom always carries its signature drum with it on its back, and pulls it out when engaged in battle. This bass drum is crafted from a tree stump, with several thick branch stumps protruding from it, which form both the legs of the bass drum as well as mid and high toms. Rillaboom also carries two drumsticks in its cuffs and takes them out to beat its drum.
By drumming, Rillaboom can tap into the power of its special tree stump. The roots of the stump follow the direction of drumming in battle. A Rillaboom with the best drumming technique can become the leader of its troop. Due to its kind disposition, it values harmony within the group. Rillaboom is very sensitive towards discord that could threaten the group's harmony, so it keeps a strict watch for such actions. Due to its strong sense of unity, it will ferociously defend its allies from trouble.
As Gigantamax Rillaboom, its upper body fuses with its enlarged tree-stump drum. The stump changes to resemble a full drum kit. The leaves expand to become four arms, each holding a drum stick while its single vine-like hair stands up and curls at the tip. As with all Gigantamax Pokémon, three red clouds circle above it. Rillaboom controls the drumsticks to perform transcendental drumming techniques. It is compelled to drum due to its power building up from inside. The sounds it makes can reach across oceans and other regions. Those who hear the beat will start to dance uncontrollably.
Rillaboom is the only known Pokémon capable of using the move Drum Beating and the exclusive G-Max Move G-Max Drum Solo.
Rillaboom may be based on a gorilla with elements from drummers, most notably Taiko drummers. Its behavior may reference the fact that gorillas make drumming sounds by beating their chests to show social dominance. It may also take inspiration from the belief that music stimulates plant growth. Its ape-like appearance may also be partially influenced by the extinct Gigantopithecus, a prehistoric genus of apes.
Rillaboom's Gigantamax form seems to draw inspiration from modern rock drummers.
Rillaboom may be a combination of gorilla and boom.
Gorirander may be a combination of gorilla and 乱打 randa (battering).