Tadeka is a veteran detective with the Fukuoka police. His death acts a catalyst for many of the events in the Mayor Harada Scandal arc of the first light novel.
Background
Some time prior to the start of the series, Takeda had begun investigating Mayor Harada's ties with the yakuza. He paid a club employee to take a photograph of the mayor with Zhang, a member of the Kakyuu Association, and caught the attention of both the syndicate and the mayor's hired killers. Knowing that he was being targeted, he anonymously mailed the photo to his junior, Shigematsu.
Synopsis
Mayor Harada Scandal Arc
At dinner with his companions, Munakata relays information about Takeda, and Ivanov immediately offers to strangle him. A short time later, Zhang orders Xianming Lin to kill the detective and sends him the detective's home address. When he arrives at the apartment building in Hakozaki, however, the exterior is crowded with police cars and onlookers gossiping about Takeda's apparent suicide.
Shigematsu is upset by his senior's death and pays the forensics team for photographic evidence of Takeda's murder. He asks his friend Zenji Banba, a private detective, to look into the matter, and gives Banba the photograph of Mayor Harada with Zhang. Banba's brazen investigation soon puts a target on his back as well.
Due to Zhang's refusal to pay Lin the advance payment for the detective's death, Lin goes on strike and chooses to protect his next target, Zenji Banba, from other killers sent by the Kakyuu Group.