Mr. Sinister - Rank 1 in the House of X's leadership and an artificial mutant. Named Nathaniel Essex, Sinister was originally a sixteenth century alchemist who discovered the X-Gene while experimenting with a mixture of alchemy and mysterious technology found off the coast of iberia. Using this knowledge, he eventually artificially activated his own mutation, which gave him what could be called a strange "jack of all trades" power, a level of super strength, a level of regeneration, a level of energy projection, a level of super speed, a level of shapeshifting, a level of psychic power, etc. but not any of them especially excelling. While high tier, these powers are not omega level or anything close to it. Ever since, he's continued researching mutants and their history and secret origins, becoming the most well versed scientist on mutation in the world, far outstripping either Zander Ryce or Moira McTaggert in any way. His personally is quite vain and narcissistic, not on the same "I'm a fucking god" level as his master, but still very high, believing himself to be a genetically perfect being and that high strength mutants like him and his fellows are the legendary master race folks like Hitler like to ramble about. He also has a somewhat humorous side, being easily the second most snark-prone member of the House other than his master, though his snarks are much lighter and less vindictive than hers. He has six daughters - Sage being one of them - each with an extremely powerful and unique mutation. Because of this, he highly values and respects them, but only as extensions of himself and his own legacy. Being a scientist obsessed with the X-Gene, he loves researching and experimenting on powerful mutants, and has a tendency to be extremely respectful to mutants who he believes are "above all others" like him, but mocking and cruel to any mutant he deems weak and inefficient.

He runs a powerful corporation, Essence, which has stock in most industries and power over even entire countries from the shadows, rendering him effectively untouchable. If anybody insinuates he's sickly or weak or doomed to fall to stronger mutants or even just exceptionally pale, he'll go berserk and kill them horribly. He believes concepts like gender and mental age and race and national identity are illusions created by a limited, superstitious species and as such can change himself to be a child or woman as he pleases, but often takes the form of a man due to how it's his public appearance.  Like his master, he wears elegant clothing, but his is usually more ornate than hers, with gold designs and fancy patterns, and always an abundance of black and red, with a common diamond theme. He also often wears a hat, unlike Cassandra. His hair is slick, droopy, and black, and his unshaven face looks beautiful and somewhat feminine, while his skin is pale. A glowing red diamond appears on his forehead when using his powers. His eyes are red with diamond shaped pupils.