To English-speaking audiences, he is known for playing tragic hero characters, such as General Tadamichi Kuribayashi in Letters from Iwo Jima and Lord Katsumoto Moritsugu in The Last Samurai, for which he was nominated for the Academy Award for Best Supporting Actor.

Among other awards, he has won the Japan Academy Prize for Best Actor twice, in 2007 for Memories of Tomorrow and in 2010 for Shizumanu Taiyō. He is also known for his roles in director Christopher Nolan's Hollywood films Batman Begins and Inception.

He is also set to star in the 2014 reboot of Godzilla.

Watanabe was born in Koide, Niigata. His mother was a school teacher and his father taught calligraphy.Due to a number of relocations for his parents' work, he spent his childhood in the villages of Irihirose and Sumon, both now part of the city of Uonuma, and in Takada, now part of the city of Jōetsu.

He attended Niigata Prefectural Koide High School, where he was a member of the concert band club, playing trumpet, which he had played since childhood.

After graduation from high school, in 1978 he aimed to enter Musashino Academia Musicae, a conservatory in Tokyo. However, he had never received a formal musical education, and because his father had collapsed when Ken was in junior high school and was unable to work there was difficulty in finding the money for tuition. In part because of these problems, Watanabe gave up entering the conservator

After graduating from high school in 1978, Watanabe moved to Tokyo to begin his acting career, getting his big break with the Tokyo-based theater troupe En. While with the troupe, he was cast as the hero in the play Shimodani Mannencho Monogatari, under Yukio Ninagawa's direction. The role attracted critical and popular notice.

In 1982, he made his first TV appearance in Michinaru Hanran (Unknown Rebellion), and his first appearance on TV as a samurai in Mibu no koiuta. He made his feature-film debut in 1984 with MacArthur's Children.

Watanabe is mostly known in Japan for playing samurai, as in the 1987 Dokuganryu Masamune (One eyed dragon, Masamune) the 50-episode NHK drama for which he is now best known. He played the lead character, Matsudaira Kurō, in the television jidaigeki Gokenin Zankurō, which ran for several seasons.

He has gone on to earn acclaim in such historical dramas as Oda Nobunaga, Chushingura, and the movie Bakumatsu Junjo Den.

In 1989, while filming Haruki Kadokawa's Heaven and Earth, Watanabe was diagnosed with acute myelogenous leukemia. He returned to acting while simultaneously undergoing chemotherapy treatments, but in 1991 suffered a relapse.

As his health improved his career picked back up. He co-starred with Koji Yakusho in the 1998 Kizuna, for which he was nominated for the Japanese Academy Award for Best Supporting Actor.

In 2002, he quit the En (Engeki-Shudan En) theatre group where he had his start and joined the K-Dash agency. The film Sennen no Koi (Thousand-year Love, based on The Tale of Genji) earned him another Japanese Academy Award nomination.

In 2006, he won Best Lead Actor at the Japanese Academy Awards for his role in Memories of Tomorrow (Ashita no Kioku), in which he played a patient with Alzheimer's Disease.

Watanabe was introduced to most Western audiences when he was cast in the 2003 film The Last Samurai by prominent Japanese film producer Yoko Narahashi.

His performance as Katsumoto earned him an Academy Award nomination for Best Supporting Actor.

Watanabe appeared in the 2005 film Memoirs of a Geisha, playing Chairman Iwamura. That same year, he also played Ra's al Ghul's decay in [[Christopher Nolan]]'s Batman film reboot, [[Batman Begins]].

In 2006, he starred in Clint Eastwood's Letters from Iwo Jima, playing Tadamichi Kuribayashi. He has voiced Ra's al Ghul's decoy in the Batman Begins video gameBatman Begins video game]].

He has filmed advertisements for American Express, Yakult, and NTT DoCoMo.

In 2004, he was featured in People Magazine 's 50 Most Beautiful People edition.

In 2009, he appeared in The Vampire's Assistant.

In 2010, he co-starred in Inception, where he stars as Saito, a mark-turned-benefactor businessman of the film's heist team.

On March 13, 2011, he launched a YouTube page to raise awareness about the 2011 Tōhoku earthquake and tsunami and invited celebrities to add their videos. In his video in English, he made a call to action to support the victims and to raise funds in the relief effort. In conjunction, he has created his own website for the cause

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