Brador was once a Hunter who was forced to kill his compatriot when they became a cleric beast. This act drove him mad, and he wore the beast's hide as a testimony to what he had seen. When he confronted the Healing Church with what he had learned, they gave him a special bell and ordered him to hunt down any hunter's who sought the secrets of the Church.
Brador is a Healing Church Assassin known as the “beast-hide assassin” trapped within the Hunter’s Nightmare. He was once a foreigner who came to Yharnam and joined the Hunt, becoming a Hunter for the Church. But when one of his compatriots transformed into a Cleric Beast he was forced to kill them. Doing so appears to have driven him to madness. Afterwards he skinned his ally’s scalp and hide and wore the scalp as a testimony to what he had seen and done. When he brought this testimony to the Healing Church they provided him with a single soundless bell of death and tasked him with ensuring their secrets would be kept. This indicates that the Scourge of the Beast originated in the Fishing Hamlet, and the curse that Byrgenwerth brought upon itself by their actions there. Prior to the events at the Fishing Hamlet, it may be that the Old Blood only had beneficial effects on those who took it, or perhaps only transformed abusers of the blood into smaller, less dangerous beasts. But this is just speculation.
Brador hid himself away in a cell below the Healing Church Research Hall and assumed his role as a Church Assassin. From his cell he rung his bell, keen to the intrusions of curious hunters. The Hunters are linked by the resonance of bells with special encoded timbres. This bell given to Brador allowed him to become the enemy of a hunter in another world and carry out his duties as assassin. We know that the Sinister Bell is an object of dark thoughts. Brador must have been driven by a single purpose, to ensure that what he knew would never be visited upon another, if he could help it.
Brador’s bell appears to react to the presence of hunters within the Fishing Hamlet. His invasions will happen again and again until the real Brador, locked away in his cell beneath the Cathedral, is killed. Simon, the Harrowed is the one who gives you the key to Brador’s cell, and perhaps intended to end the life of Brador himself, but was overcome by Brador’s relentless invasions before he could.
Brador's purpose and that of Lady Maria align. She guards the top of the Astral Clocktower, protecting the secret of the Church. She is also familiar with the blood arts of Cainhurst as shown by her techniques in the boss battle. This is important because of what we know about Brador’s weapon; Bloodletter. Finally, she was certainly aware of the Church’s secrets, and was a leader from the beginning of the Healing Church. This puts her in the likely position of being the one to give Brador the purpose which he clung to so desperately.
Brador’s actions indicate that he was feverishly afraid of becoming a beast like the one he slew. Hiding away from the Hunt ensured that he didn’t become tainted with beast blood by slaying beasts. His Bloodletter, a unique weapon and likely the weapon he originally hunted with, though it may have been given to him by the Church, was used to ritualistically ‘cleanse’ himself of tainted blood. To do this he plunging the sharpened points of the weapon into his midriff. Brador believed that by forcing himself to bleed from the inner reaches of his body and soul that the tainted blood would be expelled. His weapon grows in power when this occurs, similar to the Chikage or weapons used by Lady Maria and Yharnam, Pthumerian Queen. This makes sense considering that to strengthen a weapon by upgrading it we knead solidified blood into it. Brador must have taken this response from the weapon to mean that the tainted blood was being expelled, perhaps being fed into his weapon. Isolated in his cell, Brador had no means of furthering his knowledge about blood and its effects, and so continued to believe this to the end of his days.