The soul controls emotional memory, personality, desires, and emotions. If a character's soul is damaged, some or all of those things may be erased, although they can develop replacements over time. If their soul is completely destroyed, it cannot recover.
Souls that have been eaten continue to hold much of their own identity, and can, in response to the eater's senses, express emotions that the eater is aware of. If an eater has its soul eaten, all of the souls it has eaten are transferred with it. Being digested does cause souls to blend slightly over time, gradually taking on more and more of the attributes of the other souls in the eater's soul-stomach.
Creatures whose souls have been eaten begin to develop new souls, being essentially catatonic for the first week aside from the faintest desires for food, drink, air, and sleep, but gain more personality over time until they are essentially a new person after a year. They temporarily stop being psychic for the first week.
When a creature dies, normally its soul (and any soul it has eaten) travels to the afterlife after about 1d6 minutes. Since souls do not have any form of memory other than the emotional kind, it's very difficult to determine what the afterlife is actually like, but those who come back to life after long periods of time often return with strange psychological changes, like an acute fear of garlic, a perpetually cheerful disposition, or a significantly more vicious temper.
Souls begin at conception, and grow to full size over the course of the next year. Partially grown souls are not strong enough to reach the afterlife.