Web fiction
Self-insert
A literary device in which the author writes themselves into the story under the guise of, or from the perspective of, a fictional character.
Self-insertion is "a literary device in which the author writes themselves into the story under the guise of, or from the perspective of, a fictional character," one who "behaves like, has the personality of, and may even be described as physically resembling the author or reader of the work." The lineage is respectable: Dante walks through his own Divine Comedy, Cervantes appears in Don Quixote, Vonnegut wrote himself into his novels, and Stan Lee's Marvel cameos ran on the same device. The article notes that among professional writers deliberate self-insertion is often viewed as unoriginal.
In fan fiction the form is common, pairing an inserted reader-character with a fictional one. The craft risk is the Mary Sue: a self who is adored, always right and cost-free. The cure is what saves any character, flaws that cost something and change across the story.