Soulmates

Soulmates is a concept in Plato. A modern version of it is called Twin Flames. Mouravieff refers to it as Polar Beings.
A soulbond, more usually just called the bond, or bonded, is a very popular trope in fan and professional fiction. It overlaps with the Soulmate trope. Soulmates are two (or more[1]) characters who are fated to be together, sometimes through multiple lives[2] and/or into the afterlife.
Based on actual folklore and spiritual beliefs common to many world cultures, soulmates have been a common trope in mainstream fiction, romantic and otherwise, as well as fanfic, for decades, and is actually canon for some fandoms; for example, the TV series Lois and Clark: The New Adventures of Superman included serial reincarnation of the main characters as soulmates through multiple lives, with the time traveling H.G. Wells using a device to trace them and send the characters' souls back to their earlier bodies. Some versions of the canon for Hawkman/Hawkgirl , such as the TV series Legends of Tomorrow, also use the reincarnation version of this trope. Soulmates can be very similar to soulbonding (a mystic or psychic bond between two or more people), but soulbonding can happen through mystical means later in life, as opposed to soulmates who are always meant to be together.
See Soulbond (Fictive Presence) for the concept related to the connection between writer and character.
In the Twilight series, werewolves "imprint" on their mates in a type of soulbond that instantly engenders protectiveness, possessiveness, and (future) sexual attraction, even when there are great disparities in age, such as between Jacob and Renesmee, Bella's newborn daughter. The trope of imprinting has spread into various fandoms. Psychic Wolves are a somewhat common type of story involving people being bonded to wolves. In Magic: The Gathering, soulbonding specifically describes something that creatures, not people, do under certain conditions.[2] 
In Lois and Clark: The New Adventures of Superman there is a canon soulbond between Lois Lane and Clark Kent, and a time travel episode shows earlier incarnations as a masked gunfighter and saloon girl, a medieval maiden and a Robin Hood-like outlaw, etc.
See offsite: «Polar opposites or polar beings» at CassWiki; «Polar Beings» at Quinto Camino.
== R ==
Soul; The mind