Brain Storm manifests as a colony of extremely tiny burr puzzle-esque Stands, about the size of a penny. Members of Brain Storm have spiked feelers on their surfaces that can slightly extend further, to penetrate the surface of skin and seemingly for locomotion.
When Brain Storm begins to devour its victim, the puncture wounds appear as gaping holes on their body.
Brain Storm is a bacterial-like Stand that Urban Guerrilla sends out of his hands, which can melt the flesh of anyone touching it. First, they grab onto the surface of the skin with their feelers. They break through the skin and their feelers extend inside the victim's body. The Stand causes hemolysis and enters deeper into the body, while also constantly multiplying exponentially. This causes the target's body to break down as if holes are being dug into it. The holes that the Stand creates in its victim's body have the appearance of cryoconite holes in glaciers.
As Rai Mamezuku points out, once the Stand has made its way into the body it is impossible to remove, requiring immediate amputation to prevent death. However, the Stand is easily washed away with water if it has not entered the body yet.