This is Nocturna,and she is a Blaze-Wither Skelton Hybrid. She was born when her parents died side by side in Battle,and their essences/soul fragments came together to create her.
Like most blazes,water hurts her,and like most Wither Skeltons,she can give you the Wither effect (hence the black flames). Nocturna was the name the humans gave her when they saw her come to the overworld,and how she always preferred night over day. She spends her time traveling between the Nether,Overworld,and End,acting as some sort of Wandering Trader,but with actually good stuff for a decent price.

Pros of Being a Wither Skelton-Blaze Hybrid
Can fly (sort of)
Swim in lava
Nether mobs don't agro to her (are peaceful,with the exception of Piglin Brutes,Hoglins,and Zombie Piglins (if attacked))
Wither effect
Hostile Mobs neutral

Cons of being a Wither Skelton-Blaze Hybrid
Water hurts
Iron Golems hate her
Tall,can't fit in 2 block tall tunnels (much like her dad)
Will die if she gets too cold.
Snow hurts
Spawns in Nether.