Takashi Nasujima is a teacher at Raira Academy, and also a pedifile. He would talk to Anri Sonohara, but as soon as Masaomi Kida would show 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. Takashi hasn't been heard from since.
History
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 replotted after finding the real Saika) to appease his (Nasujima's) debt to the Awakusu, but was cornered by the Black Rider (who was issued the job by the Awakusu). However, before the Black Rider was able to retrieve the case, the Slasher appeared and attacked the Black Rider. Takashi realized his moment to escape, dropped the suitcase, and ran away yelling "Monster!".
The next night Takashi is informed by Izaya Orihara about where Anri lives 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 to extort money but that she forgave him. 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 was then that the Black Rider appeared and he 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 that debt had already been repayed 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 the it. Takashi, realizing who Anri was, then grabbed Haruna's disgarded 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 and Nasujima ran screaming. In the end, in desparation he tried to mug Shizuo Heiwajima and was subsequently pounded.