Wildfires are understood as just damage from burning trees, but they actually have a terrible 'hidden character.'
Literal
Wildfire [topic-は], trees [subject-が] burn only [possessive-の] damage [quotative-と] understood-[passive-されていますが], actually outrageous 'hidden-character' [subject-が] exists.
山火事 'wildfire.' 隠れキャラ 'hidden character' — a gaming term (hidden playable character) used metaphorically for an unexpected aspect. The quotes signal the playful metaphorical use. とんでもない 'outrageous, unbelievable.' The sentence teases a surprising revelation about wildfires.