Takashi Nasujima

Takashi Nasujima (那須島 隆志 Nasujima Takashi) was a teacher at Raira Academy. He was attracted to Anri Sonohara and would talk to her when they were alone, but as soon as Masaomi Kida showed up, Takashi would leave.

Before he targeted Anri, he was in a relationship with Haruna Niekawa. When he learned about Saika, he spurned Haruna and she moved to a different school. However, she later returned to pursue her love for him. Before she could cut him with her Saika, Anri appeared to save him. After Takashi found out that Anri controlled the real Saika, he ran off. He was last seen desperately trying to mug Shizuo Heiwajima, but the incident ends as expected, with Shizuo knocking Takashi out. Nasujima reappears in Ketsu (Volume 12-13), as a main antagonist of the last arc, after harnessing control of Saika.

Saika Arc
Takashi was shown to frequently sexually harass Anri Sonohara at school but was usually interjected by Masaomi Kida, who would scare him off. Masaomi also seemed to know a bit about Nasujima's previous affair with Haruna Niekawa. When he recorded a video of Takashi harassing Anri and threatened to show the video to the other teachers, Takashi eventually stopped.

One night Takashi was seen running with a case full of money stolen from Izaya Orihara who had invited him over to extract information about the Niekawas for the Awakusu and baited the cash in the first place so that he could capture him and blackmail him into using Haruna Niekawa for his (Izaya's) own plot, afterwards re-plotted after finding the real Saika) to appease his (Nasujima's) debt to the Awakusu, but was cornered by Celty (who was issued the job by the Awakusu). However, before the Black Rider was able to retrieve the case, the Slasher appeared and attacked Celty. Takashi realized his moment to escape, dropped the suitcase, and ran away yelling "Monster!".

The next night Takashi was informed by Izaya Orihara about where Anri lived and proceeded to visit her. When he got to the house, Haruna, however, happened to be there too, and Takashi ran from the residence. He was later cornered by Haruna who went on about being used by him to extort money but that she forgave him. Haruna then sliced her hand and rubbed it on Takashi's face. Just as he was about to be slashed in an extreme show of love, Anri cut in and pleaded with Haruna to stop but was attacked herself instead. Anri in the end defeated her and Takashi explained that Haruna had tried to cut him once before and he had her transferred to another school. It is then that the Black Rider appeared. Takashi tried to convince Anri to escape with him saying that she owed him for getting her and Mika Harima into the same class. Anri then explained that debt had already been repaid when she had suspected him of being the next victim of the Slasher (whom she believed to be the Black Rider) and protected him the night he encountered it. Takashi, realizing who Anri was, then grabbed Haruna's discarded knife in self-defense. Anri went on to assure him that she would not slash him out of Saika's love, for she rather detested him. Nasujima ran away screaming. In the end, he tried to mug Shizuo Heiwajima in desperation and was subsequently pounded.

Light Novel Volume 13/x2 Ketsu
After a long absence, Nasujima reappears prior to the events of the Tokyo Hands incident, first speaking to Hiroto Shijima disguised, claiming to work for Jinnai Yodogiri. Previously fired from Raira after being left comatose for a month by Shizuo Heiwajima, Nasujima is bent on avenging the actions against him by his fellow Ikebukurian citizens. After being cut by Kasane Kujiragi, he has become a Saika host, but overcame Kasane's control and was able to form his own rogue faction. He was able to gain control of Haruna's father, using him to incapacitate Haruna so he could kidnap her and assume leadership of her Saika pawns, as well as to have a sinister brand of "fun" with her. He later stabs Mikado while trying to attack Anri. Following the attack, he is kidnapped by Haruna. His status is unknown from this point on.

Durarara!! SH
While he doesn't appear, it is revealed that Mikado is a member of the Saika Army thanks to him.