Brax

Bix is a hero from Haven, a city famous for producing strong heroes. Bix is a Hero Arbitrium, awarded the honor after finding and returning stolen goods to a local family. He is currently under consideration for the Laureate rank after capturing the serial arsonist of Feyfell (a town in Wellby that borders the fey region). 
The fastest and most sure-fire way to become a hero is to enter the The Academy of Heroes in Haven, which is what Bix did. He applied on his eleventh birthday and was rejected. He applied again on his twelfth and just barely made the cut. The Academy is an intense, five-year program that guarantees the rank of hero upon graduation. It teaches all skills necessary to be a successful hero ranging from the Hero's Code to combat to survival magic. However, there is one major flaw to the Academy. It only admits boys. 
Brax's real name is Bella. Her brother is Brax. Or rather, was.
Bella's parents were simple city administrators. They were comfortable, paid by the taxes of the people, but not notable enough to warrant any real attention. They had two children, Brax and Bella. Brax was a small boy. He loved magic. He wanted to be an enchanter when he was older. Too young to officially begin the study of magic, Brax spent a lot of time reading about it in books.
Bella liked the outdoors. She liked to wander the parks near their house and search for bugs. She liked to wrestle the other boys in the neighborhood. Her parents were worried about a young girl doing such unfeminine things. It wasn't a problem now, since she was still a child, but if they didn't find more feminine hobbies it could affect her public appearance and appeal when she became a woman of society. They tried to encourage her interest in things like painting, language or cooking, but Bella hated them. 
Bella's parents often had to attend various public functions, including the graduation at the Academy. When Bella was eleven and Brax eight, they were going to meet their parents at the event. They entered the Hall of Heroes' entryway only to be pushed out again by a crowd of officials. They were holding a woman who was screaming about the injustice of the system. She demanded the right to be a hero. She was escorted away and the officials of the event began pouring out from the Hall, including their parents. Bella asked why that lady couldn't be a hero. They said women can't be heroes. Why not? It's just the rules. Bella didn't like that. Weren't heroes supposed to help people and care about justice? Bella decided she wanted to be a hero. 
Bella decided to apply for the Academy despite the "no girls allowed" rules. She wasn't even allowed on the venue when her day of observation came. She tried again the following year. They wouldn't even look at her application, knowing it was a girl. She vowed to find a Laureate hero who would support her. She couldn't find one.
Her parents had allowed Bella to make these attempts, knowing they wouldn't succeed and hoping that Bella would accept reality before long. She didn't. 
Bella was fourteen when their house caught fire. It was minor, but caused structural damage. Brax was caught inside and died. He was eleven. Their family secluded themselves from the world as they mourned. Now the future of the family came down to Bella, who was proving to be quite a troublemaker. Her parents feared she would have a long and lonely life if she couldn't accept the way of the world and start thinking about marriage and family.
Bella knew their fears and devised a plan to make everyone happy. A week before their mourning was to end, Bella presented her proposal. What if Bella had died and not Brax? Brax could live on and carry the family name. He could find his wealth through the career of his choice and they wouldn't have to worry about Bella never marrying. What if Bella became Brax? She could enroll at the Academy and become a successful hero. Her parents were hesitant, but knowing their daughter's fighting spirit it did seem like a reasonable option. They agreed. 
Bella, now Brax, entered the Academy, being careful to hide her secret. Luckily, Brax had been a bit shy, so it wasn't out of character for him to shower and change privately. Despite her secret femininity, which her mother helped her conceal via enchantments and potions, Bella had great promise. It was difficult, especially keeping a secret, but she managed to graduate.
Bella loves her profession, but she still holds a grudge that she never could have realized this potential if it wasn't for her brother's sacrifice and she can't even honor his memory appropriately because the world thinks he's alive. After three years as a hero, Bella has achieved a fair amount. It comes with some money and privilege, but she doesn't like to be idle long enough to enjoy them. Her mind can go to dark places and can be consumed by guilt if she rests too long. It eats at her.
Bella/Brax decides it is long enough and she wants to be Bella again. She wants to prove to the world, especially the Academy, that girls CAN be heroes. But if she revealed her secret now, she wouldn't put it past them to revoke her status and send her back to a life of limitations. But, the ultimate rank, Gran Deo, could hold enough leverage and immunity to persuade them otherwise. So Bella began preparing for the task of killing a Lorenthum.