Otakukin are members of the otherkin community, who identify as something that is considered a fictional character(s).
Most Otakukin 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 a character(s) could live, die and then be reborn in a different universe, which explains some fictionkins spirituality. This would also explain memories or déjà vu experienced while reading, watching, or listening to something that is fictional.
The multiuniverse theory also explains why many people might identify as a certain character(s). The character(s) could have existed in many different universe, therefore being reborn multiple different times into multiple different people.