Hiroto Shijima

Shijima is (technically) the second in command of a drug cartel that primarily deals with a very unstable and powerful drug called "Heaven Slave" (for simplicity's sake we will refer to the group as Heaven slave as well). He is one of two major antagonists introduced in volume 09 of the light novel series along with Earthworm.

Dragon Zombie Arc
Before the start of the series and during his college years, Shijima was part of a "Legal drugs" club that was started by Izaya and Nakura (It is unclear as to what the actual activities of the club entailed due to it only getting a passing mention at the end of volume 09). Shijima proved to be very proficient at mixing and experimenting with different substances which would further cement his role as the eventual leader of the Heaven Slave group.

After he had graduated from college he had formed Heaven Slave and immediately began experimentation for a new drug. It is unknown how long it took Shijima to perfect it although the drug was known to be in limited distribution since about five years before the start of the series and was even partially responsible for the deaths of the parents of Anri Sonohara. In order to protect himself from incrimination, Shijima decided to set up Nakura as a scapegoat/leader of Heaven slave while Shijima played the role of an obedient second in command. He used one of Nakura's aliases "Kumoi" as the name of the "leader" and did what he could to make "Kumoi-san" seem real even going so far as to inflict injuries on himself when his plans weren't successful to make it seem like "Kumoi-san" was punishing him.

By the start of Volume 09, Heaven Slave had become much more widely distributed and Shijima's cartel (along with another group "Amphisbaena") was gradually starting to encroach on the territory of the Awakusu. Wanting to avoid an all out gang war on two fronts, Shiki hires Izaya Orihara to dig up what he can about Heaven Slave as well as "Amphisbaena." After digging up what he could, Izaya leaves a laptop with most of the information he collected on both groups with Celty. Shijima and his men were spying on both of them at the time and kept tabs on Celty the entire night. The next day, when Celty was distracted, Shijima's men snatch the laptop and take it back to their hideout to analyze it. Shijima finds the location of Amphisbaena's hideout and by extension, the place where they were keeping Izaya for interrogation.

Shijima sends some of his men to tail Mairu Orihara while he and a small entourage barge into the gambling parlor where Earthworm was interrogating Izaya. Shijima demands that Earthworm hand Izaya over with the intent of killing Izaya for all that he knew.The two briefly argue over what to do with Izaya when Earthworm gets a call from someone claiming to be the "Real Izaya." While both Shijima and Earthworm are dismissive at first, they become panicked when "Izaya" hints at the person under the bag being not only the wrong man but is actually either Earthworm's or Shijima's boss. Earthworm hastily loosens the restraints when another group of people barge into the room.

Man they had tied up begins laughing and reveals himself to be Izaya himself and that the person on the phone was Ran Izumii. Izaya goes on to explain that he allowed himself to get kidnapped in order to lure both groups that he was investigating into a corner. After easily dispatching Amphisbaena as well as Shijima's henchmen, Izaya coerces Shijima into joining forces with him, saying that he would have access to the dollars information network as well as avoid all out conflict with the Awakusu. Shijima tries to threaten Mairu to get back at Izaya but he is shot down when Izaya states that he hired Celty to protect his sisters in case of this eventuality.

Kadota's Coma Arc
Shijima is on the run after having been cornered by Izaya. He heads home. His father and grandfather are implied to be powerful and rich, especially his grandfather, who knows a politician. He plans to use his family to take revenge on Izaya. However, when he gets home, Yodogiri and his secretary Kujiraki is at his house speaking to his father and grandfather, who are intimidated but sucking up to him anyway. His grandfather says he is aware of his crimes and will deal with the Awakusu, which raises his hopes, only to crush them when he is told to obey Yodogiri and not let Yodogiri down. Regarding Shijima's first orders, Yodogiri expresses his interest in the Dollars, claiming to have been interested in it for a long time.

It is revealed that Shijima's grandfather fended off the Awakusu with three hundred million funded by Yodogiri, but in return had to affiliate the family to the yakuza, and was unable to convince them to overlook Yodogiri.

Shijima later approaches Mikado as a friendly Dollars member who wants to help with the "cleansing". This is presumably under the request of Yodogiri. Mikado is said to be unaware of Shijima's past and his bitter desire to bring as many people as he can down with him.