Hi, I'm a new user and was hoping that I could get some help concerning a few characters in a plot I have. There's a couple characters in that story who are immortal through a substance in their bodies that humans can't identify, but I don't really know how to explain how they became that way. The thing is, they aren't exactly main characters, so I don't want to go too deeply into their pasts other than how they cause a few of the conflicts in the main plot. I was thinking that maybe I could leave their exact origins a little unknown so it can be left up to the readers, but I'm a little worried that it might end up sounding like a cop-out. Thanks.