In Minecraft, the Dark Forest is a biome in the Overworld. It has lots of dark oak trees and is easy to spot because of the large red and brown mushrooms towering through the trees.

My thought process behind creating this character is, that at a very young age they were abandoned in the forest and have come to adapt to the forests resources such as:

-weaving clothes from the giant mushrooms roots.

-using the small mushrooms that grow on the ground as a primary food source like mushroom stew, as well as eating the non poisonous brown mushrooms.

-And eating the glow lichen that grows in caves to make themselves have a faint glow as a portable light source so they can see during the night.
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As this character continued to eat the mushrooms and wear the weaved mushroom root clothing, the mushrooms themselves started growing on the person, then soon after this was used a camouflage against hostile mobs.
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Over time while the person was living in forest letting the shrubbery and foliage brush against them and fall onto themselves more and more, it soon merged with their skin and made it permanently green, which wasn't bad since it helped to be be another way of camouflaging them from mobs.
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by allowing these plants to stain their skin and grow on them they became one with the forest themself and never wanted to leave, they feel safe, and at home here, they can hear what the trees whisper when the wind blows past, when they dance along the paths and trails they have known for so long they can see the shrubbery and small plants growing upon the ground dancing with them, singing a sweet lullaby that only they can hear.
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Hope this was a good enough description. :D