Posthuman refers to a concept in science, fiction, and everything in between. A posthuman is a human being whose physical and/or mental capabilities are enhanced via technology. Taken literally, a posthuman this can be anything from a superman upgraded with genetic engineering, or a person compensating for flawed vision with glasses. Of course, the latter is only posthuman in the most literal possible sense of the word, similar to how Michael Bay can ostensibly described as a filmmaker.

In the context of the Ruby Saga, posthuman is a subjective label for any human being enhanced by nanomachines. This does not include characters using technology created by nanomachines, or even characters whose abilities come from an unexplained source (such as Alexis' ESP). All posthumans are the result of the nanotechnology developed by Dr. Tori Carcharria and Dr. Evelyn Sloan-Harrison. The nanotechnology edits the subject's genetic makeup and triggers mutations towards the desired form. However, the procedure has a slim chance of success with all but a select few who bear a compatible set of chromosomes; for anyone else, the procedure results in a quick, painful, and messy death. The subject must also be in good physical health, given the extreme stress caused by the procedure. Dr. Carcharria's need for subjects was met through kidnapping people identified as genetically compatible, usually younger people. The immoral nature of this work, involving abduction, vivisection, and brainwashing, led Dr. Sloan-Harrison to launch a secret counter-initiative, remaking her daughter Ruby as a post-human powerful enough to stop any of Dr. Carcharria's super-soldiers.

Posthumans in the Ruby Saga include;