Kōki Tanaka (田中聖) (b. November 5, 1985 in Chiba, Japan) is a Japanese rapper/singer-songwriter, actor, television personality and radio host. He joined Johnny & Associates in 1998 and became a member of the popular J-pop group KAT-TUN in 2001. Kōki is the second T in the group's name. The group officially debuted in 2006 and has since achieved an unbroken streak of success with all five albums and thirteen singles debuting at number 1 on the Oricon charts.

A senior to the other KAT-TUN members, Kōki disliked them and wanted to leave the group by his own admission when it was formed but changed his mind after the group held their first concert. Influenced by hip-hop and reggae since childhood, he credits 櫻井翔 (Shō Sakurai of 嵐) for making a breakthrough for rap in the agency and making his task of rapping in an idol group easier. Kōki writes his own rap verses in KAT-TUN and his own solo songs respectively under the pen name, JOKER.

Artistically inclined, he can illustrate well, often creates or re-modifies his own apparel and even designed a pair of sneakers for Ne-Yo which the latter wore during his performance at the 2007 MTV Video Music Awards in Japan. With bandmate 中丸雄一 (Yuichi Nakamaru), they make up a comedic unit, TaNaka (a portmanteau of their surnames), and perform skits together during concerts. An actor who won multiple awards for his performance in My Boss My Hero, he has also appeared in Byakkotai, Tatta Hitotsu no Koi and took on his first lead role in 2007 in Tokkyu Tanaka 3 Go. He recently starred in the 2009 season of TV Asahi's Hissatsu Shigotonin as Ren, an assassin, with senpai 東山紀之 (Noriyuki Higashiyama of 少年隊 [Shōnentai[) and 松岡昌宏 (Masahiro Matsuoka of TOKIO). He is also the older brother of Johnny's Jr member, 田中樹 (Jūri Tanaka).

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