Death, usually preceded by a gendered honorific — Lady or Mistress — is Marvel's embodiment of the universe's inevitable decomposition. She is an elemental being, which means that she is part of the fabric of reality (which ironically binds her fate to that of the cosmos).
Although she has not had much of a role to play in the various Marvel storylines, the times that Lady Death has been included have centralized her character to the extent of placing her on a literal pedestal (by her incel-like admirer). The list provided below elucidates a few details about Death that few people are aware of.
10 Death Was Never Born
With the "creation" of the universe by the One-Above-All, most of its cosmic entities spontaneously emerged from the newborn reality, bearing various aspects of time, space, emotions, life, and so on.
Death is included among Infinity, Oblivion, Mistress Love, The Phoenix Force, and others, but her diametric opposite is Eternity, the personification of existence and being. As such, Death has no use for cosmic artifacts/devices, as she herself is her own power, being the manifestation of the concept of death, or non-life.
9 Her Form Is Her Choice
Death takes the form of a human woman (or a bizarre skeleton with breasts), but this is mostly because she tries to maintain the same appearance as those observing her. This eases a soul's passage beyond the Veil, especially if she chooses to appear to the dying as one of their friends/family/lovers.
Interestingly, Death's anatomy is actually constructed by a strange species of "Living Fractals," which combine and recombine in limitless ways and provide cosmic beings a physical-ish shape.
8 She Is Ineffable
Death cannot be measured, the same way that there is no way to quantify darkness or cold (except through the converse concepts of light and heat).
Therefore, while several of her "siblings" are motivated by specific desires, Lady Death performs her role with the utmost efficiency, all the while refusing to divulge any of her plans, whether she has them or not. This stone-cold exterior — and interior, to be honest — is quite suitable for her personality, as Death isn't meant to have favorites in any case.
7 Her Relationship With Thanos
Thanos is a supremely powerful being; one who has achieved great things, depending on the perspective, on more than one occasion. All of this megalomania begins, however, upon his first encounter with Death while living as a child on Titan.
Death exploits the Mad Titan's Deviant-ness, offering her love in exchange for him becoming the greatest destroy in the history of the cosmos, which he does (at first, to win her favor, but he quickly develops a taste for his new persona). Unsurprisingly, Death wants death more than she wants Thanos, as the next entry will explain.
6 She Is The Reason Behind The Infinity Gauntlet
In the comics, the core of the entire Infinity Gauntlet storyline surrounds Death (and Thanos' obsession with her). She tells the Mad Titan to assist her in restoring balance, as she senses that life is teeming at a rate much higher than death. Hence, Thanos suggests that he go on a hunt for the six gems, whose abilities combined would let him materialize any wish.
After his job is complete, Thanos performs the renowned snap, deleting fifty percent of all extant organisms, but Death still won't address him directly.
5 Death Is Forced To Break Her Vow
Galactus is attacked by the Elders of the Universe, but he succeeds in defeating them (even swallowing five of them in a clear breach of Death's promise to them that they will remain immortal forever).
In a later scene, the In-Betweener comes into the fray and tries to fling the Devourer of Worlds through a black hole, an act which is stopped by the Trader, Possessor, and Astronomer. Considering the threat posed by Galactus, the In-Betweener calls on Death to stop the three Elders, forcing her to break her own vow.
4 She Has A Thing For Deadpool
Deadpool and Death have what can be best described as a long-distance romance — he cannot die, and she cannot be with him until he does. Their relationship is quite intense on both sides: Deadpool makes out with a random skull believing it to be his beau, and Death excitedly waits for him to come and save her from Eternity's overconfidence.
What most interesting, though, is that Deadpool might be a funny guy, but Death has proven herself to be his match in almost every way possible. It's just funnier to watch an abstract entity make "That's what she said jokes."
3 Death As A Mother
Thanos dies, as he has a habit of doing every now and then, but this time his "spirit" blends with that of his Mistress', which produces their only known child, to use the term loosely, known as The Rot. It is called so because of its tendency towards utterly destroying anything in its vicinity, whether living or dead.
The new parents are not gleeful about their firstborn, which is not a total shocker given that it could go on to end creation if not controlled. Therefore, they work together and figure out a way to erase The Rot from existence.
2 She Is "Killed" By The Beyonder
The Beyonder has been, in various avatars, an outerversal being whose capabilities far exceeded the power cosmic of the Marvel Universe. Once he's here, though, he tries to protect humanity from dying by somehow exterminating Death herself, which causes a different set of problems entirely.
Thankfully, the Molecule Man is around to persuade the Beyonder to rebuild the metaphysical being, claiming that life has no meaning if death also doesn't exist to provide perspective.
1 Death Really Doesn't Give A Hoot
It is unlikely that even the likes of the Infinity Stones can cause Death any lasting harm; the fun thing about being an abstract entity means she will exist as long as things experience life. She is dedicated to her goal, which is not the taking of life, but actually the maintenance of equilibrium across the cosmos.
Death doesn't abide by things like morality, honor, integrity, virtue, courage, and other such silly notions invented by humans. She is an unbiased chaperone, ferrying souls across the ether to wherever they are to go next.