He made his acting debut in the television series Gokusen in 2002 and made his first film appearance in Bright Future in 2003.
He starred in the film Winning Pass a year later.
2006
He is known for his affinity for strange character roles, and he is best known internationally for playing L in the 2006 films Death Note, Death Note 2: The Last Name and L: Change the World in 2008.
2009
Matsuyama starred in Ultra Miracle Love Story in 2009, using the Tsugaru dialect of Japanese in the film.
2010
He was cast to play lead character Toru Watanabe in the film adaptation of Haruki Murakami's novel Norwegian Wood, which was released in December 2010.
His stage name uses the katakana spelling of his first name (ケンイチ) instead of the usual kanji (研一).
He graduated from Tokyo Metropolitan Toyama High School.
2011
On April 1, 2011, he married Koyuki Katō, who co-starred with him in Kamui Gaiden.
2012
The couple's first child was born in January 2012, and their second child was born in January 2013 in South Korea.
2015
In July 2015, the couple had their third child.
He speaks in the Shimokita dialect.
2017
He voiced the role of Balthazar Bratt in the Japanese dub of Despicable Me 3 in 2017.
He was in the stage play Seven Souls in the Skull Castle the same year.
He played a police detective in the 2023 drama Why Didn't I Tell You a Million Times?
