The phenomenon you're referring to is known as "fictional character delusion" or "fictional identity syndrome." It is a rare psychiatric condition in which an individual holds a strong and fixed belief that they are a fictional character from literature, movies, TV shows, or other forms of media. This type of delusion is considered a subtype of delusional disorder, which is a mental illness characterized by persistent false beliefs that are not based in reality and not culturally accepted.
People experiencing fictional character delusion may firmly believe they are the character in question and may exhibit behavior consistent with that character. They may also have difficulty distinguishing between their own identity and that of the fictional character. It's important to note that this is a psychological condition and requires professional diagnosis and treatment by a qualified mental health professional.
As with any mental health condition, the specific cause of fictional character delusion is not entirely understood, but it can be related to underlying psychological factors and may be triggered or exacerbated by stress, trauma, or other mental health issues. Treatment typically involves psychotherapy and, in some cases, medication to address any underlying psychiatric conditions. If you know someone who may be experiencing this or any other mental health issue, it's essential to encourage them to seek professional help from a mental health expert.