'Fictionkin' (formerly called 'otakukin' or 'mediakin') are those who identify as something that is considered fictional, typically a fictional character(s) or species. Characters may not always be directly mentioned in the canon of their source material; for example, an original character (also known as an oc), a background character, a noncanon character, or occasionally even a glitch.
If you are looking for a definition of relating to something that is considered fictional, with a level of self-projection onto the fictional character whether it be through headcanons of sexuality, trauma, disability, gender identity, etc., having a strong and/or deep connection with something fictional, choosing an identity for coping reasons, etc., check out the pages, Copinglink, Other-hearted, Otherlink.
Some 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 common 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 are made very uncomfortable by other people who are 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. Though this behavior and attitude is often discouraged in fictionkin communites, as it is rude and and contributes to fights and fragmenting of groups.
Spiritual explanations for being fictionkin are just as diverse as those for otherkin and therians. Arguably the most common belief is past lives and reincarnation, where one was their fictotype in another life. Though this is far from the only explanation, there are also misplaced or walk-in souls where the soul of a fictional entity wound up in a different or the wrong body, soul parts, where aspects of the soul are of a fictional entity, and soulbonding, where a fictional entity bonded with a host.
There are many, many different ways a fictionkin might identify spiritually as their fictotype. Some are more common, some are less common. Fictionkin and otherkin as whole is not a unified spiritual belief, and psychological explanations aside there are so many different spiritual beliefs.
Though identify as fictional species has been around for as long as the otherkin community has, fictionkin got its start as a separate community as otakukin during the early 2000s. The term otakukin comes from the now defunct website Temple of The Ota-’Kin. Though 'Otaku' is associated with anime and Japanese media, the term was inclusive all who identified as entities from fictional sources.
The earliest use of the term fictionkin comes from the From Fiction LiveJournal created in 2004. Though otakukin continued to be widely used, by around the mid-2000s fictionkin started to be more widely used, before the term otakukin fell into more obscurity in the late-2000s.
Notable events in the fictionkin community include the Final Fantasy House which was an IRL cult centered around fictionkin and soulbonding, and the Yandere Bitch Club which is sometimes considered to be why the Tumblr otherkin community is so different from the rest of the otherkin community.
Many people (both within and outside of the otherkin belief/culture) are critical of fictionkin, claiming it to be "taking things too far", "roleplaying", or "being a copycat." These statements are typical towards the otherkin grouping at large, but more so to fictionkin than other kintypes. There are also criticisms of stealing from the author .
There is some debate of if fictionkin is considered otherkin or not. Though under the kin umbrella, the definition of otherkin is identifying as something nonhuman, and with fictionkin there are those who identify as human characters. Some believe they are always otherkin, some believe they are sometimes times otherkin, and some believe they are never otherkin.