Nine Kurosu is a blader who appears suddenly before Team Persona, the team that has risen to the pinnacle of the X City professional beyblading scene. She demonstrates skill comparable to Masked X, the alias of Jaxon Cross, and is revealed to be a member of the influential Kurosu family. Specifically, she is identified as the sister of Kurosu Ex, also known as Jaxon Cross. Her arrival is tied to a warning about the Kurosu family endangering the professional system of X City, though her true intentions remain ambiguous throughout her early appearances. She joins Team Persona after seeking their help to confront her father, Zero Cross, who stands at the center of the threat.

Nine Kurosu possesses a deep familiarity with the advanced technology developed by the Cross Corporation and also holds a Beycrafter qualification, indicating expertise in designing and customizing Beyblade parts. Her fighting style uses her Beyblade ValkyrieVolt S4-70V, and her battles reflect the high-level capabilities of the Kurosu bloodline. In terms of personal development, she transitions from an enigmatic figure who tests the protagonist team to an active member who collaborates with them against a common enemy. Her relationships are centered on her brother Jaxon Cross and the wider Kurosu family conflict, which drives much of her motivation. Her role in the story is to introduce a new crisis for the professional league and to serve as a bridge between the established protagonists and the looming threat of her father and the rest of the Kurosu clan.