In the
Marvel Universe, Death (who is more commonly referred to as Mistress Death) is one of the so-called
abstract beings that embody various
aspects of the universe. This means she is in the same class as
Eternity,
Infinity,
Oblivion, Lord
Chaos and Master
Order, etc. Her face was first revealed in
Captain Marvel #31, but let's face it, even in comics, Death has always been around.
She is always seen wearing a purple cloak, and generally takes two forms. She is seen either as a female or a skeleton, which is more in tune with the traditional Death image. She is always silent, preferring to speak through her minions. And of course, she is responsible for every death in the Marvel Universe.
As a general rule, the writers use her well, rather than oversaturating stories with her. She embraced Captain Marvel when he died of cancer, and met him on the other side. She is often seen standing passively by during universe-threatening conflicts, such as The Infinity Gauntlet, or when Adam Warlock confronted the Magus.
Perhaps the only mortal she gives any sort of special consideration is Thanos, who loved her. She actually brought him back from the dead in an improved form so that he might do her bidding. Still, she maintained her silence with him, which infuriated the Titan. When he obtained omnipotence with the Infinity Gauntlet, he thought she might finally speak to him since they would be equals. That was not the case, for the Infinity Gauntlet had actually made him her superior, and as such, she would not address him out of respect. As of now, Thanos doesn't much care for Death, but some writers love that angle quite a bit, so it may be resurrected at any time.
As far as other mortals Death has affected without killing them, Deadly Earnest kills everything he touches now because he spurned Death, so she made it so that he could never touch another living thing.
Death is easily one of the best characters in the Marvel Comics roster, one that writers can't mess around with too much, because Death is constant.