- المَوْتِ, عزرائيل or Azrael is the angel of death and one of the four horsemen of the apocalypse. He presents himself as a humanoid male who answers to the name of "William" which ultimately results in the nickname "Billy" within the series.
- Death is exceedingly kind, understanding and compassionate for humans, speaking in a very soft, gentle and somewhat troubled voice. Death is quite passive and can behave statuesque at times and has a refusal to participate or become involved in human affairs.
- Death appears to be an Ethiopian male with a trim, fit body and broad shoulders. Death wears a fearful or sorrowful but largely blank expression and is perpetually crying; wearing both wet tears and dried salt trails where tears have been. His sclera appear jet black with white, blue tinted irises that lack pupils. Death has pure white hair in loose curls that are longer on top but trimmed along the back and sides. Death is surrounded by dozens of floating eyeballs and shards of blue crystal that hover in midair and can sometimes take the form of other objects entirely, such as glass shards and angelic wings.
- The same blue floating crystals that appear before the arrival of the Gwydr also appear when the angel of death makes an appearance.
- Percy is one of the only known individuals to be able to see and interact with the angel of death without being under the specific circumstances otherwise necessary for such encounters. (I.e. dying, or being born.)
- Death introduced himself by a human name to Percy which happened to be 'William.' Perhaps because William is a European and English name and Death was speaking English and in Europe at the time. Percy promptly renamed him Billy.
- Death is the only character who implies any religious, non-human higher power or deity-like beings have any hold or place in the inner-workings of the story. Death is one of the only characters and phenomena that are not explained with science or pseudoscience in either series, left purely to biblical reasoning. Death's very existence as a character challenges the almost strictly scientific laws of the overarching universe.
- The author has stated Death is modeled after an Ethiopian man because what is now Ethiopia is considered the birthplace of modern humans and "death of man has existed as long as man."
- It is not known if Death guides only the souls of humans or if Death takes the form of animals to guide them as well. It is not known if there is a Death for each species, or only for species who can conceptualize and understand dying.
- Death is present whenever human beings die or are born into the world.