Introduced in volume 4, Aoba is 1st year student in Raira Academy. He has a bright personality and is always seen smiling, but has also a darker side to him. He's described as being akin to "a younger version of Orihara Izaya."

When he was in elementary school his older brother Izumii Ran physically abused him, so Aoba in revenge set out fire in Ran's bedroom and Ran took the blame, as the adults thought the fire started from his cigarette. After their parents' divorce, Aoba went to live with his mother and changed his surname from Izumii to Kuronuma. A few years later, he formed Blue Square and ceded its leadership to Ran.

When Blue Square was almost disbanded after various major conflicts with Yellow Scarves and then Heiwajima Shizuo, Aoba returned and took over the control. He enrolled at Raira academy and approached Anri and Mikado, whom he knew to be a Dollars member. Later, he desired to use Mikado for his own purpose and declared Orihara Izaya, whose influence on Mikado interfered with his plans, an enemy. He successfully persuaded Mikado to become the leader of Blue Square; however, when Mikado assumed control by stabbing Aoba's hand with a ballpoint pen as his first order, Aoba realized there was more to Mikado than meet the eyes.