Nathaniel Essex, also known as Mr. Sinister, is a fictional character in the Marvel Comics universe. He first appeared in The Uncanny X-Men #221 in 1987. Essex is a geneticist with a deep obsession for evolution and genetic manipulation, often experimenting on mutants to achieve his goals. His character is known for his intelligence, cunning, and mastery of genetics. Essex has a complex history, often associated with the X-Men and their adversaries. He is a key figure in the Marauders and has connections to other characters like Cyclops and Jean Grey. His ultimate aim is to create a superior race of mutants, making him a formidable foe in the ongoing battle between mutants and humans.