With a combination of famousness,willpower and ego, the consciousness of humans (which is also a kind of Information), the very concept of a singular human can manage to imprint himself into the Semiosphere, achieving a kind of semi-immortality. But only if the Ego, that is, the sense of self of that person, is very very strong can he retain a conscious, sentient existance. Otherwise, he or she will simply disperse and become a tiny speck of information in the gigantic flow.

However, the existance as part of the semiosphere is a very confusing, incoherent one, defined mostly by the constant need to reaffirm you still exist as you, as there are no actual limits to mind and body, after all, your body doesn't exist anymore and your mind is not an individual entity but part of many other informations.

So time as an imprint flows by in very imprecise patterns, not static at all, and you don't really perceive many changes of the human world while you exist like that.

But eventually, Thickening may occur: The Semioplasm of the Semiosphere overflows, usually via conceptual valves.

Most of the time by Semioplasmats (Those who can shape/control Semioplasm). Imprinted Personalities will always try to get out of the actual Semiosphere and into the real world, where their minds will be cut off from other mental patterns and the sheer mass of information and they can retain individuality easily.

They also will have a new body: That of the Semioplasmat.

Even though Possession sounds rather like dominating, the Host and the Imprinted Personality (the Ghost, actually) have more of a symbiotic relationship. They help out their hosts with advice and take part of their personal experiences. Of course there are cases where the ghost DOES try to take over the individual, but usually, the Hosts are very strong personalities already, and extremely hard to take over so they will fail. And failure will mean they probably will not be able to go back to their existance as Imprints on the Semiosphere, instead, disappear completely.

A fate most wouldn't dare to risk.

But that's not all: Having existed as part of the Semiosphere, the ghosts have developed quite some skill in shaping Semioplasm themselves, and add that to the power of the host. They also took along the ideas that have come to be associated with their person since their deaths, something the Hosts can shape into formidable weapons. (This is why, for example, Aya can use attacks resembling aeroplanes and aeronautic maneuvers, and is not just able to fly like Manfred von Richthofen could)

Additionally, as willpower and strength of believe into one idea have great power over Semioplasm and the Semiosphere, they fact that Host and Ghost combined have more willpower and ego in one entity than a normal human is an extreme advantage in combat as well.