Lady Death, sometimes referred to simply as Death, is a Marvel Comics cosmic-entity and the personification of “death”: although she is seldom seen and rarely speaks she has been known to manipulate and interfere with mortals on a few occassions: amongst her would-be-lovers are Thanos, Walker, and even Deadpool- to Thanos she "mothered" the anomaly known as Rot and she teamed up with one of Thanos' Thanosi clones to destroy this offspring before it could devour the universe.
Although often content to keep a balance between life and death she is shown as more than happy to take advantage of situations so as to get the upper-hand: Lady Death is also known to empower most death-gods and demons, allowing them to take souls that have died though the exact nature of their dealing is unknown.
Lady Death is technically without gender but prefers to take the form of a female skeleton dressed in robes - though s/he has appeared in other forms, such as when Death faced Dracula as a male warrior when his creations of vampires was seen as a threat to Death's own power.
Relationship With Marvel's Villains
Although Death is a neutral entity by nature she isn't above trying to seize control over things and has a rivalry of sorts with her "brother" Eternity - though Lady Death is wise enough to understand the need for balance and as a result rarely engages in acts of outright villainy, instead she can be seen as the ultimate mastermind who observes creation and allows it to grow while at the same time formulating ways to expand her own influence over it.
Lady Death was temporarily erased from existence by the omnipotent alien known as Beyonder, but was returned when he realized that destroying Death went against the very nature of existence and as such, even with his great power, Beyonder was not right in his actions.
Death's relationship with other villains has varied, she is most famous for her "romance" with Thanos - though she often appeared to shun her would-be-lover, perhaps playing him as somewhat of a fool as her rejection made him ever more determined to prove his worth. In a cruel twist of fate, Thanos once obtained absolute omnipotence in the form of the Infinity Gauntlet only to find that Death still would not address him, this time due to the fact that Thanos was superior to her.
Although often seen as cruel in her actions towards Thanos Death may well of had reason to distance herself from his advances as it was shown that once she and Thanos merged they gave "birth" to the entity known as Rot: which was neither dead nor alive and thus beyond even Death's capability to control.
Thanos was not the only villain who wished to win Death's affections, the death-god known as Walker once tried to impress the entity via exterminating all life in his native galaxy. However, such a massive loss of life was seen as shameful by Death as it threatened order, this embittered Walker and he began a feud with her, at the end learning the hard way that Death is also a being that despises cruelty.
Although normally a female, Death has taken the form of a human male to fight Dracula, claiming his creation of vampires threatened her influence - since a world of undeath would, logically, be a world without true death - unsurprisingly Death won this battle.
Death also came to collect the soul of Deadly Ernest, yet he rejected her and was transformed into a villain who could kill with a touch as a result.Lately Death has been shown to have active interest in Deadpool. He is one of the few - if not the only - "sane" mortals that can see and interact with her. During his torture after his failure in Department K, she would often distract him with fantasy worlds for the two, such as dancing together or having a picnic. She was the reason he wanted to die during that time, so he could be with her. However, when Deadpool activated his healing power through his need for vengeance, he could no longer see or hear her which upset the plans she had for him. While they had a flirtatious relationship, she made it clear that they would never so much as kiss until after he died.Thanos, jealous of her attention and affections toward Deadpool, makes sure they can never be together. In his efforts to keep them apart, he gives Deadpool's worst enemy, T-Ray, the power to bring Deadpool back to life after he was killed by Weapon X. Thanos also gives T-Ray a cosmic item, the Gemini Star, which would split Deadpool's personalities apart from him, leaving him nothing but a boring shell, unappealing to Death. Even though the plan failed, Thanos cursed Deadpool with a worse fate than death; with life, preventing Deadpool from ever enjoying Death's embrace.
Marvel vs Capcom
Lady Death was approached by Jedah Dohma, who convinced Lady Death to help join him and complete his plan to merge the Marvel and Capcom universes together, to help bring forth the equilibrium between life and death. Lady Death entrusted her faithful servant, Thanos, with the space stone, who then gave it to Ultron to help them get the Reality Stone that is beyond a wall, only penitrable to him.
At the end of the game, after their plan failed and Ultron Sigma was destroyed, Lady Death was seen having a conversation with Jedah, who was telling her there is another way. Just then, Thanos came in with Satsui no Haido, after he realized that Lady Death used him. Lady Death tells Thanos that no power can harm her. However, Thanos tells her only power from her universe can harm her as he uses Satsui no Haido to conjure an energy ball. It was unknown if Thanos destroyed her or not.