'Fictionkin' are members of the otherkin community, who identify as something that is considered fictional, typically a fictional character(s), though that character may not always be directly mentioned in the canon of their source material. (Ex. An OC, a background character, occasionally even a glitch.)
People who have chosen, of their own volition, to take on the identity of a fictional character usually use it for a coping mechanism of some kind. They are called copinglinkers, or C-linkers, and have been the subject of much debate within the fictionkin community.
Most fictionkin believe in the multiverse theory. They believe there are limitless parallel universes, which means there are ones where fictional characters are real. If this is true, then theoretically a character could live, die, and then be reborn in a different universe. This is a good enough explanation for most fictionkin's spirituality. The aforementioned would also explain memories or déjà vu experienced while reading, watching, or listening to something that relates to the fictional characters.
The multiverse theory also explains why many people might identify as certain character. The characters could have existed in many different universes, therefore being reborn multiple different times into multiple different people. Some fictionkin however, particularly those with identity issues, are made very uncomfortable by other people who are kin with the same characters as them. Those who do not believe that a character existed in anymore than one universe, and when the character was reborn, they where the only one to become that character could also feel this way.
Many people (both within and outside of the otherkin belief/culture) are critical of fictionkin, claiming it to be "taking things too far" or "being a copycat." These statements are typical towards the otherkin grouping at large, but more so to fictionkin than other kintypes.