He was abused by his mother the first few years of his life, along with his older siblings, Until his father finally divorced her and moved out. He was a very intellectual yet anti-social baby, always locking himself up in his room, which greatly annoyed his father. He was very spoiled, despite his father having a limited amount of money.
His anti-social behaviors continued through his childhood, much to his father's dismay. His adopted brother, Sanderson, became part of the family early into his childhood, and he met his future wife, Anti-Wanda,during middle childhood-ish. He became slightly more social,but not by much.
Anti-Cosmo was kicked out by his father when he was in his late teens, his behaviors being too much for his aging father to handle, He ran away with Anti-Wanda and they lived like blue hobos for a while. He killed the leader of the Anti-Fairies at the time when he was just finishing his teen years, and took control of Anti-Fairy World.