Time for some rambling about Z, this time on the subject of "Z's Standards for Proper Immortality".
Z's concern, with immortality, is that he does not want to go to Hell. In settings where there is no Hell/dead people don't go to it, and this is known, he will not seek immortality. I don't know what a Z in such a setting would be like, nor do I know what a naturally-immortal one would look like.
Zs differ from Bells in that Zs care not at all about whether anyone else gets immortality. He will make no particular effort to stop it becoming widespread unless it is legitimately a scarce resource for some reason, but the only person he cares about becoming immortal is himself. This can be changed if e.g. he magically falls in love with someone, but it's a fairly sticky default.
One of the major problems for Zs is that they have Standards for their immortality. They don't want it to be the kind where a version of you dies but a version lives on, because that means they still have to die and go to Hell even if there's another version of them walking around. They also want it to be them, not an approximation thereof with their memories. They object to Sunnyvamps and daeva on these grounds. Daevafication especially messes Zs up because they are demons, and ending up in Hell despite his best efforts is not good for Z's sanity.
They get kind of ridiculously paranoid about this, including rejecting anything that even looks like a Ship of Theseus issue. Uploads don't qualify as Proper Immortality either, and neither does cloning unless you get fancy. On a slightly more positive note, forms of immortality that are acceptable include the Philosopher's Stone in Potterverse, any kind of magic that can just "make you immortal", e.g. anti-aging necklaces, and golden apples from Narnia. If you would like to submit a form of immortality from glowfic for Z's consideration, leave a reply below or catch me on Discord/IRC between 11 am and midnight GMT+1.