Rillaboom is a bipedal Pokémon that looks like a gorilla. Its body is mostly various shades of brown: light tan on its forehead, chest, abs, and limbs; dark brown on its nose and triangular ears, as well as under its eyes; and a medium, mud-like brown covering the rest of its body, including its robust arms and legs. Its mouth, the tip of its nose, and its irises are orange, while its sclerae are white. Each upper arm has tan marks resembling bow-ties or butterflies, and it features two diamond-shaped, tan marks running down its spine. The larger mark starts around its shoulder blades, and the smaller one begins below its rib cage.
Rillaboom sports a thick bush of green, leaf-like hair on its head, which flows down its back. A single vine-like hair, resembling a music note, falls between its eyes and curls to the left of its nose. On each hip, there are green, belt-like adornments that look like four leaves in a row. Similar decorations with two leaf-like parts sit near its clavicles on each shoulder; these appear to be part of its hair when not attacking. Rillaboom's wrists have cuffs resembling leafy crowns, with the peaks facing its hands.
Rillaboom always carries its signature drum on its back, pulling it out during battles. This bass drum is made from a tree stump, with several thick branch stumps protruding from it, forming the legs of the bass drum and mid and high toms. It also keeps two drumsticks in its cuffs, taking them out to play its drum.
By drumming, Rillaboom can harness the power of its special tree stump. The roots follow its drumming direction in battle. In a troop of Rillabooms, the one with the best drumming technique leads. Challenges for leadership are settled through drum battles. Rillaboom has a kind and calm nature, valuing harmony within its group. It watches strictly for any discord threatening this harmony, disciplining offenders until they learn their lesson. Due to its strong sense of unity, it fiercely defends its allies.
Forms
Rillaboom has a Gigantamax form. As Gigantamax Rillaboom, its upper body fuses with its enlarged tree-stump drum, resembling a full drum kit. The leaves expand into four arms, each holding a drumstick, while its single vine-like hair stands up and curls at the tip. Like all Gigantamax Pokémon, three red clouds circle above it. The power building up inside Gigantamax Rillaboom compels it to perform transcendental drumming techniques, controlling its drumsticks as if they were its own limbs. The sounds it makes can reach across oceans and regions, causing anyone who hears it to start dancing uncontrollably.
Evolution
Rillaboom evolves from Thwackey and is the final form of Grookey.
