Rillaboom is a bipedal, ape-like Pokémon. Its body is mostly mud-brown in coloration, including its face and body, as well as its thick arms and legs. Its face has an orange snout, with a dark brown nose. This dark brown coloration is also seen on its triangular ears, as well as under its eyes, which themselves have orange irises and white pupils. Its forehead is a light tan color, as well as its chest, abs, hands, and feet. It also has a bow-shaped mark on each shoulder of the same color. Rillaboom also has leaves protruding from its head, creating a thick bush of hair on its head, with a single vine that falls from its forehead like a strand of hair. It also has a belt and cuffs made of leaves. 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.
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 boss of its troop. Due to its kind disposition, it values harmony with 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.
Rillaboom is the only known Pokémon capable of using the the move Drum Beating.
As Gigantamax Rillaboom, it has become larger with and fused with its enlarged tree-stump drum. The stump has changed to resemble a drum kit. The leaves hair expands to become four arms while its single vine become straight and standing while bending. There red clouds float above it. Rillaboom, as a result of Gigantamaxing, has become more powerful due to fusing with the drum. It can control the sticks of the drum to preform transcendental drumming techniques.
It is compelled to drum due 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 has the highest base Attack stats of all fully evolved starter Pokémon.
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.
Rillaboom may be a combination of gorilla and boom.
Gorirandā may be a combination of gorilla and 乱打 randa (battering).