He is the head of the Japanese National Police Agency.
He was kidnapped by the Mafia led by Mello in order to serve as a hostage in exchange for the Death Note the Japanese police had in its possession. Takimura was later found dead, strangled by his own tie. Mello therefore organised the kidnapping of Sayu Yagami, the daughter of Takimura's deputy Soichiro Yagami, and held her in exchange for the Death Note.
Death Note: How to Read 13 confirms that he was killed by Kira (Light Yagami), though in the manga Light himself does not openly confirm this in his thoughts or in conversation. Near and Mello themselves speculate that it could have been either Kira's doing or actual suicide.