Tetsuya Arai (新井 哲也, Arai Tetsuya) is the former information technology teacher and the system administrator of the school's information network at Teru Kurebayashi's school. During his time as a teacher, he was involved in a relationship with Rena Ichinose and was adviser to the Student Council. However, he used his position to embezzle funds from the school's budget until Teru solicits DAISY's help in order expose Arai's corruption.
Arai is fired soon after being exposed, but is forced to become a pawn in a series of illegal activities targeting DAISY by Chiharu Mori. Out of concern for Rena's safety, he approaches Teru to disclose information that would expose the organization coercing him. The incident results in Teru's abduction, during which Arai entrusts Teru with a USB containing data related to the recent hacking incidents perpetrated by Chiharu Mori. However, in order to save Arai, who has been injured and requires medical attention, Teru leaves the USB with him while she leads Mori away from the area. Arai is found by the Master while Tasuku Kurosaki goes to save Teru.
Arai is subsequently hospitalized for his injuries, but is otherwise able to recover. He cooperates with Kurosaki and Kazumasa Andou in providing what information he knows regarding Chiharu Mori, Akira, and why they were targeting DAISY and Teru.