Multiple Selves - The ability which allows to have several different entities existing within one being. They can either being minds, souls, living organisms or even combinations of these types. This ability has a number of advantages and a number of shortcomings.
Multiple selves are classified in the following manner:
Type 1 (Multiple Minds): Generally describes those who has one or more personas or personalities co-inhabiting their mind. Note that characters that simply suffer from a dissociative identity disorder (also known as multiple personality disorder) don't normally qualify for this ability; in order to meet the requirements, their alternative identities must be extremely relevant, influent and grant them exceptional or supernatural powers, abilities and/or resistances, sometimes even affecting the very structure of their body.
Type 2 (Multiple Souls): Generally describes those who have one or more spirits or souls co-inhabiting their body. Those who are possessed by one or multiple spiritual beings would fall under this category. In order to qualify as "selves" such spiritual beings should preferably be sentient, regardless of them being autonomous, dormant or forcibly controlled.
Type 3 (Multiple Organisms): Generally describes those who have one or more organisms co-inhabiting their body. This includes parasites, symbiotes, multiple heads and such.
- Examples: Venom (Edward Brock) (Marvel Comics), Shinichi Izumi (Parasyte), Ryūko Matoi (Kill la Kill)
- The division of roles - Each self can be strong in some ways and is only activated when there is a need for its services.
- Collective Intelligence - One head is good, two is better, and if one head has a million consciousness at once, with a sufficient level of organization, they can replace a strategic headquarters or scientific institute.
- Change in body - Switching between selves may also change the body, getting different (and likely necessary) skills and abilities.
- Mental resistance - The presence of a large number of consciousness allows resistance to mental influences, because even if one of them succumbs to external control, the other personalities immediately bring the wayward personality to its senses.
- Disorganization/chaos/power struggle - If each self pulls control towards itself, a situation may arise in which control of the body randomly shifts from one mind to another, which obviously does not benefit the whole person.
- Increased complexity of control - Usually if there is one dominant self, it has to constantly monitor all the other consciousness, which requires considerable effort, and in certain situations severely limits other abilities (because there is no mental direction at the other faculties of the body).
- Problems with memory - Because the selves are still autonomous, when you switch from one to another, the memory information may be lost. Or else, one of the personalities may unknowingly erase the memories that are important to the another.