Koki Uchiyama is a Japanese actor and voice actor born on August 16, 1990, in Saitama Prefecture. He is affiliated with the Himawari Theatre Group, a talent agency he joined at the age of three in 1993, which marked the beginning of his career as a child actor in television dramas. Uchiyama graduated from Waseda University's School of Culture, Media and Society in 2013, balancing his higher education with his growing professional commitments.

Uchiyama's transition to voice acting began in his youth, with his first voice work being for the Japanese dubbing of the film A.I. Artificial Intelligence. His profile rose significantly when he was selected to voice Roxas in the popular video game Kingdom Hearts II in 2005, a role that led to further opportunities and recognition. His first lead role in a television anime came in 2008 as the voice of Soul Eater Evans in Soul Eater. This period also saw him take on other prominent roles, such as Banagher Links in the Mobile Suit Gundam Unicorn OVA series, starting in 2010.

Uchiyama is known for his distinctive voice, often described as having a natural, slightly deep timbre with a subtle rasp. This quality has made him a frequent choice for characters who are introverted, emotionally complex, or possess a "shadowed" nature, a description famously applied to him by Kingdom Hearts director Tetsuya Nomura. His acting approach favors naturalistic delivery over exaggerated, archetypal anime styles, focusing on subtle emotional expression. He has demonstrated considerable range across genres, portraying protagonists, antagonists, and supporting characters with psychological depth.

His filmography includes an extensive list of major anime roles. In sports anime, he is known as Kei Tsukishima in Haikyu!!, Ikuya Kirishima in Free! - Dive to the Future, Takashi Sugiyama in Run with the Wind, Yōichi Kiriki in Aoashi, and Rin Itoshi in Blue Lock. In fantasy and action series, he has voiced Meruem in Hunter x Hunter (2011), Tomura Shigaraki in My Hero Academia, Toge Inumaki in Jujutsu Kaisen, and Akira Fudo in Devilman Crybaby. His other notable performances include Izumi Miyamura in Horimiya, Raku Ichijou in Nisekoi, Yū Otosaka in Charlotte, Yuri Plisetsky in Yuri on Ice, and Philip in Fuuto PI. He has also voiced roles in popular films, such as Gin in Hotarubi no Mori e.

Beyond anime, Uchiyama has a significant presence in video games, reprising his role as Roxas and also voicing Ventus in the Kingdom Hearts series. He is also known as Neku Sakuraba in The World Ends with You, Rean Schwarzer in The Legend of Heroes: Trails of Cold Steel, and V in Devil May Cry 5. His work in other media includes dubbing foreign content, such as providing the Japanese voice for the character Sam in the video game The Last of Us.

Uchiyama's talent has been recognized with several industry awards. He won the Best Male Rookie Award at the 5th Seiyu Awards in 2011. He was also a recipient of the Best Voice Actor Award at the Tokyo Anime Award Festival in 2015.