Hiroto Shijima (四十万博人, Shijima Hiroto) is introduced as the second-in-command of a drug cartel that primarily deals with a very unstable and powerful drug called Heaven's Slave. Hiroto is one of two major antagonists introduced in the ninth light novel.
For simplicity's sake, the group is here referred to as "Heaven's Slave" as well.
As the creator of Heaven's Slave, he is responsible for certain events in the series, specifically, the ones involving Anri and Saika.
During his college years, Hiroto was part of an "Experimental Legal Drug" club started by Izaya and Nakura. Hiroto proved to be very proficient at mixing and experimenting with different substances. This would further cement his role as the eventual leader of the Heaven's Slave group.
After graduation, Hiroto formed Heaven's Slave and experimented until he successfully created a new, potent drug. The drug was in limited distribution since approximately five years before the start of the series, and would eventually be responsible for Anri Sonohara's father's drug addiction.
To protect himself from incrimination, Hiroto framed Nakura as the leader of Heaven's Slave and pretended to be the group's second-in-command. Hiroto played the role of an obedient second in command. He used one of Nakura's aliases, "Kumoi", as the name of the "leader" and would go to great lengths to prove "Kumoi" was real – even going so far as to inflict physical injuries upon himself when his plans failed as 'proof' that "Kumoi" was punishing him.
Dragon Zombie ArcEdit
By the start of volume nine, Heaven's Slave was experiencing a growth in distribution and Hiroto's cartel was starting to encroach on the Awakusu's territory.
Wanting to avoid an all out gang war on two fronts, Shiki hires Izaya Orihara to compile information on both Heaven's Slave and Amphisbaena, a rival drug-based group. Izaya later leaves a laptop with most of the relevant information with Celty; not only do Hiroto's men spy on both individuals, Hiroto himself and a few followers spend the entire night keeping personal tabs on Celty.
The next day, Hiroto's men steal the laptop while Celty is distracted and take it back to their hideout to analyze it. As a result, they discover the location of Amphisbaena's hideout - where Izaya is currently undergoing interrogation.
Hiroto sends some of his men to tail Mairu Orihara, and he and a small entourage head to into Amphisbaena's hideout in person. They find Izaya bound to a chair, his head obscured by a bag; Hiroto demands that the interrogator Earthworm hand Izaya over, planning to kill him, and the two argue over what to do until Earthworm receives a call from someone claiming to be the "real Izaya." Both she and Hiroto are at first dismissive, but panic when "Izaya" hints that the captured individual may in reality be either Earthworm's or Hiroto's boss. Earthworm loosens the restraints seconds before another group of people barge into the room.
The bound man starts laughing, revealing that he is the real Izaya after all; the man on the phone had been Ran Izumii, the leader of the newcomer group. Izaya goes on to explain that he allowed himself to get kidnapped as a means of baiting both Heaven's Slave and Amphisbaena. After his people dispatch both Hiroto's and Earthworm's followers, he coerces Hiroto into joining forces with him: he claims that Hiroto would have access to the Dollars' information network and at the same time avoid all out conflict with the Awakusu. When Hiroto attempts to use Mairu as blackmail, Izaya reveals that he preemptively hired Celty to temporarily act as Mairu's bodyguard.
Kadota's Coma ArcEdit
Hiroto flees for home, intending to use his family's power and connections to take revenge on Izaya. However, when he arrives, he finds Yodogiri and his secretary Kasane Kujiragi in the middle of conversation with his father and grandfather. Both men are intimidated by Yodogiri, and Hiroto's grandfather Giichiro orders him to obey Yodogiri from now on. In the meantime, he and Yodogiri fend off the Awakusu with three hundred million yen and subsequently affiliate themselves with the yakuza.
During Mikado Ryuugamine's 'cleansing' of the Dollars, Hiroto offers Mikado his assistance under the guise of a friendly, unassuming Dollars member – likely under orders from Yodogiri, who had previously expressed interest in the Dollars' organization. Mikado suspects nothing, unaware of both Hiroto's past and Hiroto's bitter desire to 'take down' as many people with him as he is able.
He is revealed to be the one to cause the headless rider cult to be founded and provided the Heaven's Slave drug to them. He is determined to destroy Ikebukuro and partners up with Earthworm to exact his revenge on the city. At some undetermined point of time, he witnesses Jami take out local gang members that infiltrated the casino they were at. Impressed with his skills, he has Jami work as one of his muscle men and seems to form some kind of friendship with him.
A few months later, he determines Anri is one of the original Saika wielders and approaches her, demanding for her to sell her Saika or for her to cut him and make him a child of Saika.