If /x/-tan is the girl to talk to for all your creepypasta, cryptids, and conspiracy theory needs, Fringe Girl is the one who'll teach you how to protect your body against demonic possession while you're astral projecting. Her focus is on the study and practice of magic, and while she'll gladly help anyone interested to learn the craft, she has little to no interest in anything outside its scope. Nonetheless, she's very well-read and has tons of experience in her field while still constantly pushing herself to improve as a magician, which makes a valuable source of help for anyone looking to learn more about magic. Sadly, due to most people's unwillingness to take magic's existence as serious as she does, she tends to come off as delusional and haughty, which estranges her from most of the rest of the site.
There isn't much history to how Fringe Girl came to be /fringe/'s board-tan. An artist on DeviantArt unrelated to /fringe/ created her, the board owner used her picture in the OP of a thread he then stickied, and over time the userbase grew so used to seeing her whenever they visited the board that they eventually declared her their mascot.