Brad "BB" Blackstone is a tall man who typically wears sunglasses and is a former United States Army Major who later became a member of the Brahmin, the elite warriors of the Selinistan Kingdom, before ultimately becoming an independent agent. His stated goal is to rid the world of all Spell Crests, a mission he takes upon himself after discovering the dark truth behind their origins and the motivations of those who wield them. To this end, he operates as a free agent, aligning himself with neither the U.S. military nor the Kingdom, and his actions are primarily focused on disrupting the plans of Princess Aryabhata. He is one of only four known individuals who possess the immensely powerful Void Maker Spell Crest, a fact that places him at the center of the conflict.

In the story, BB serves as a mysterious and pivotal figure who greatly influences the protagonist, Seigi. Seigi views BB as a true hero and a mentor figure, a sentiment that grows after BB kidnaps him and takes him to destroy a Kingdom-operated tattoo research laboratory in Japan. BB's methods and philosophy have a deep psychological impact on Seigi, ultimately helping to decide the direction of the narrative. Seigi later inherits BB's mission to destroy all tattoos after experiencing BB's memories, which allow him to integrate BB's combat planning and execution skills into his own fighting style.

Regarding key relationships, BB shares a romantic connection with Izzy, a member of the U.S. military's Spell Crest unit. He also maintains a cooperative relationship with a character known as Wiseman, who provided him with crucial information. Due to his departure from the Kingdom, he is branded a traitor by its warriors, such as Iltut Mishu. His combat abilities are formidable, and as a Void Maker user, he possesses a highly accelerated healing factor.