Relia Satan, formerly known as Makoto Fujimura, is the older half-sister of Shirou Fujimura, the current possessor of the Longinus, Divine Dividing, as well as the inheritor of the Will of Satan. She had raised Shirou since the incident ten years ago in which Shirou's mother, Miyako Fujimura, was killed, having altered his memories to believe he had known her since he was born.
As Makoto, she possessed snow-white hair with two long strands in braids and blue eyes similar to Shirou's. She tended to wear simple clothes such as various T-shirts and sweatpants and possessed a larger B-cup bust size. When angered, she released a demonic aura that could frighten even someone of Ultimate Class.
In her true form, Relia's hair turns midnight-black (although she does sometimes retain her white hair color) and her eyes become blood-red. She wears an elegant yet battle-ready white dress and goes without shoes.
As Makoto, she was very outgoing yet very caring and protective of her little brother. However, she possessed a violent streak which often resulted in physical standoffs between the siblings, considering it "their way of showing sibling affection". Makoto was very perverted and enjoyed teasing her younger brother concerning his romantic and sexual relationships. She also showed happiness that he was able to find love with many women. Despite this, Makoto has a limit concerning the possibility of Shirou impregnating his lovers and has a very strict rule concerning raising children, refusing to take care of more than two in the worst-case scenario.
During Volume 3, her facade slowly began to break away as Shirou slowly began to regain his sealed memories, showing a much colder and harsher side to the point that she wordlessly dragged Shirou by the neck and locked him in his room when he refused to leave without getting answers from her. It was only in Volume 4 during the Peace Treaty Conference that her mask breaks away entirely, revealing a cold, brutal, and ruthless personality focused only on completing her mission.
As Relia, she shows a deep disdain for childish antics and often scolds those around her. She also shows a deeply-seeded affection for her father, doing whatever it took to earn his love and approval. Due to her brutal and loveless childhood, Relia shows a deep envy and hatred for those who are given such feelings such as love, especially Shirou whom she truly hates deeply. Relia despises Shirou because, while she was born into a loveless world, Shirou always had the love and affection of both of his parents including their father. This jealousy sometimes leads her to acting recklessly and behaving erratically.