Coalossal resembles a massive goliath with a body of coal atop of it. It has a large, coal-black body, with two legs and a pair of stubby arms, alongside two massive spikes for elbows. It bears a head consisting of a pair of eyes and a mouth, atop which it carries a large amount of heated coal.
Coalossal has a peaceful disposition, but becomes enraged if the coal mines it lives in are vandalized, incinerating the offenders with flames as hot as 2,700 degrees Fahrenheit (1,500 degrees Celsius). In battle, the coal on its head will burn bright red and radiate sparks of heat that send sparks flying.
As Gigantamax Coalossal, it becomes considerably larger, and the mountain of coal expands, with three red clouds surrounding its summit. Its physique is largely similar to its original form, but its front now features three open spots, which expose intense heat radiating from its body, and it resembles a stove oven. Its fire can burn even hotter than 3,600 degrees Fahrenheit (2,000 degrees Celsius), and some say that it served as a massive heat stove when Galar was struck by a harsh cold wave, saving many people in doing so.