Phastis

Phasti are the counterparts to the Animuses. They represent the unconscious mind and soul from their split host. Like the Animus, there are natural and artificial Phasti, but artificial Phasti are typically harder to make than artificial Animuses.

Physiology
Phasti tend to look almost exactly like their host before splitting into Animus and Phastis, but they tend to be very pale. Artificially created Phasti can also display a range of colors not commonly found in natural Phasti, but they're not nearly as vibrant as their Animus counterparts.

Phasti, especially naturally occuring ones, are much more stable than Animuses and usually live longer lives. However, their average lifespan is still only about half of that of a healthy human.

Origins
Naturally occuring Phasti are born when their host comes into contact with powerful artifacts, usually when in an unstable state of mind. The shock of splitting into Animus and Phastis appears to affect the Phastis little, and most are able to continue on within a few minutes as if nothing had happened.

Psychology and Habits
Like Animuses, Phasti tend to act rashly, though this seems to be due to a lack of planning instead of an emotional drive. Phasti have even or heightened survival instincts to that of their host, and thus are much less likely to display the self-destructional behavior that Animuses do. A Phasti on its own will grow to be apathetic, and, at their worst, purely reactionary to their environment.

Phasti also have a very limited memory. Most do not remember much from before their split, and have a hard time recalling events since their split.

Differences Between Natural and Artificial
Artifically created Phasti are much less common than artifical Animuses. While they are easier to create initially, due to their natural stability, issues with apathy and memory keep them from being of much use outside of pure research. We can assume this is why the Darkness Requiem prefers to produce Animus soldiers.

However, it should be noted that it is still possible to create a perfectly functioning Phasti, just as it is to create stable Animuses. Unlike Animuses, though, creating a stable Phastis usually involves months or years of therapy outside the lab to keep them stable, wheras Animus stability is purely chemical.

Relations to Other Creatures
Phasti are the counterparts to Animuses, as one of each is created at the time of the host's split. Stable Phasti are able to blend into society much more easily than Animuses, as their pale but solid state stands out much less than the Animus' black and murky appearance.