He began acting as a child and is sometimes mistaken with Daisuke Hirakawa, as their names only differ by one character when written in kanji.

He has also been cast as anti-heroes or antagonists, such as Ulquiorra Cifer in Bleach, Hisoka Morow in Hunter × Hunter (2011), Kei Kurono in Gantz, Eustass Kid in One Piece, Dr. Genus in One Punch Man, Chōsō in Jujutsu Kaisen, Tōru Oikawa in Haikyuu!

He has also dubbed over some roles that were performed by other fellow actors such as: Leonardo DiCaprio, Tony Jaa, Edward Furlong, and Kevin Zegers in Japanese.

He made his directorial debut for Wonderful World, a live-action film that opened in Japan in early summer of 2010.

On July 19, 2017, Shūkan Bunshun reported that Namikawa had been involved in an on-and-off extramarital affair with a female employee from his previous agency since 2004, whom he met when she had still been a teenager.