[9] Koyamada holds black belts in Karate and Tae Kwon Do and won gold medals in Shaolin Kung Fu at the U.S. National Championships.

[14][15] In 2000, Koyamada moved to the United States to further his studies as an international student, enrolling in the ESL program at the University of California, Riverside.

[26] Koyamada next appeared in the 20th Century Fox drama Constellation (2007), with Zoe Saldana, Billy Dee Williams and Hill Harper.

[27] From 2006 to 2008, Koyamada was a regular in the Disney Channel Games, appearing with Miley Cyrus, Jonas Brothers, Selena Gomez, Cole Sprouse and others.

[30] In 2014, Koyamada starred in and produced the independent television series The Yokai King, which was entirely filmed in Okinawa, Japan, and is based on famous Japanese folklore characters.

In 2007, Koyamada produced a 20-minute short action drama film Good Soil, written and directed by Craig Shimahara and released by R.A.G.E.

[34] In 2014, Koyamada executive produced an American television series The Yokai King, directed by Yuji Makino, in Okinawa, Japan.

[36][37] In 2011, Koyamada developed and produced the original digital stickers Kolala, which were published on the Line app, a platform for instant messaging and social networking.

[38] In 2013, it was announced that Koyamada joined as an executive producer for a cyberpunk RPG video game based on the historic legendary samurai Miyamoto Musashi.

[42] In 2004, Koyamada began his support for youth empowerment by teaching karate at a local Boys & Girls Clubs of America in Burbank.

[43] Koyamada begun supporting the program by participating in the annual charity event alongside Chuck Norris and President Bush, and by inspiring youth at the middle schools they worked with.

In earlier years, Koyamada has supported various charitable organizations across the United States, including the Japan-America Societies, National Cherry Blossom Parade, Los Angeles Mission and others.

[46] KIF's global programs address a broad range of topics including youth leadership, women's empowerment, cross-cultural, disaster relief, educational in emergencies, natural conservation and space.

[58][59] Later that year, he was elected as the first Japan-born Board of Directors of the SCI since its founding of 1956, with then-Chair Ron Nirenberg, the Mayor of San Antonio, Texas.

[61] In March 2019, Koyamada created the first-ever Japan-Texas Leadership Symposium, hosted by SCI, KIF USA, and the City of San Antonio, and supported by the Ministry of Foreign Affairs of Japan.

[62] The event aimed to foster friendships between the people of Japan and the United States through sister cities, with a focus on business, culture, and education in San Antonio, Texas.

In 2000, shortly after moving to the United States, Koyamada began training in Northern Shaolin Kung Fu in Los Angeles six days a week.

[68] In 2002, during the filming of The Last Samurai, Koyamada trained daily in horseback riding and Kyudo for nine months in Los Angeles, Japan, and New Zealand to eventually master Yabusame.