Illumise is a firefly-like, bipedal Pokémon. It has a blue face and large, slanted blue eyes with purple markings at the corners. Surrounding its head are purple curls that frame its face. Its antennae are small and yellow. Its body is black with a blue belly and a yellow bid marking. Its arms and legs are black and are connected to the body by purple borders. It has small wings used for flying with others, and a pattern of three oblong yellow spots on its rear. This is a female only species; Volbeat is its male counterpart.
Illumise uses its sweet aromas to lure in Volbeat and have them draw geometric patterns in the sky. It is said to earn respect from others by composing designs that are more complex. Over 200 different patterns have been discovered. It is a nocturnal Pokémon that lives in forests.
In Time Warp Heals All Wounds, Edna mentioned how Illumise and Volbeat used to do their mating dance rituals in a large tree that used to be located in the middle of the city. In the new timeline May created after time traveling, the tree remained, and the migrations never stopped.
In Zoroark: Master of Illusions, an Illumise living in Crown City worked with several other Pokémon in surrounding Zorua at the park. They were intent on attacking him as retribution for what Zoroark did to the city. Before a fight could break out, Illumise and the other Pokémon were calmed by Celebi.
Illumise appears to be based on a lampyridae (also known as a glowworm, firefly, or lightningbug) like its counterpart, Volbeat. It does not seem to share its coloring with any particular species. Illumise may be based on flappers, a 1920s women's style. They usually had bobbed hair, like Illumise's purple "hair", and wore taboo dresses and outfits. Illumise seems to be wearing a leotard, black jacket, coat, or dress, similar to Volbeat's, with purple arm and leg cuffs. The yellow bow-like shape under its neck is reminiscent to that of a yellow Peter Pan collar and the stripes on its abdomen resemble a girdle.
Illumise may be a combination of illuminate and luciferase (class of enzymes that produces bioluminescence).