Kohama was born in
Osaka before moving to Tokyo.
Toru Yamashita, a fellow member from One Ok Rock, is his childhood friend. They formed a hip-hop dance group HEADS when they were fifth and sixth graders with Takuya Uehara and Kohei Nakamura. They were also active before their debut and participated in dance competitions around the years of 1995–1999. They debuted with a single titled "Screeeem!" in July 2000, and it was followed by the second single "Gooood or Bad!" in November, which was produced by a popular musician under the label
Amuse, Inc. The dance group made a performance every weekend on the busy streets in Osaka's Kyoubashi and Tokyo's Akihabara, shopping, and amusement districts. Heads disbanded in 2002 and in 2003 they took a break to improve their abilities. At this point, Heads as a four-member group ended. They merged into a new group with seven other members and were called Ground 0. In 2004, Toru and Ryota continued their activities as Ground 0. In 2005, when Toru tried to explore acting and took a main role in the TV series
Shibuya Fifteen, Kohama also played a minor character in the drama. That same year, he was invited by Toru to form a rock band, alongside their upperclassman Alex and Toru's friend Koyanagi Yuu. In the beginning, Kohama initially had no interest in joining a band or playing bass, until he listened to
Red Hot Chili Peppers and was inspired by
Flea to play bass. During the early days of One Ok Rock, Kohama only talked to Toru because he felt inferior and didn't talk to vocalist
Takahiro Moriuchi for about a year because he was afraid of him. He was the most furious when Alex was caught groping a schoolgirl on a subway and almost quit playing bass to learn to play guitar. Kohama collaborated with drummer
Tomoya Kanki and wrote the song "
Deeper Deeper". == Personal life ==