Early career

Riamu was scheduled to make his professional debut against Hiroaki Shinohara at Krush.87 on April 22, 2018. He won the fight by a first-round technical knockout.^[2]

Riamu was scheduled to face Kosuke at KHAOS.7 on November 17, 2018. He won the fight by a first-round technical knockout, dropping Kosuke with several well place knees and hooks.^[3]

Riamu was scheduled to make his K-1 debut against Lyra in the preliminary portion of the K-1 WORLD GP 2019 JAPAN ～K'FESTA.2～ on March 10, 2019. He won the fight by majority decision, with scores of 29–28, 29–29 and 30–29.^[4]

Move up to super bantamweight

Riamu was scheduled to face Akihiro at Krush.104 on August 31, 2019. He moved up to super bantamweight (55kg) for this bout, a two kilogram increase from his previous fights. Riamu won the fight by unanimous decision, with scores of 30–27, 30–28 and 29–28.^[5]

Riamu was scheduled to face Sho Uchida at K-1 KRUSH FIGHT 109 on December 15, 2019. He won the fight by a sensational first-round spinning back kick knockout.^[6]

Riamu was scheduled to make his second K-1 appearance against Ryuto on March 22, 2020, on the preliminary portion of the K'Festa 3 card. He won the fight by unanimous decision.^[7]

Riamu was expected to face Aoshi at Krush 113 on June 28, 2020. However, on June 7, it was announced that Riamu withdrew from the bout due to injury.^[8]

Riamu was scheduled to face the former Krush Bantamweight titleholder Junki Sasaki at K-1 World GP 2020 Winter's Crucial Bout on December 13, 2020. Sasaki won the fight by unanimous decision, with scores of 29–27, 30–26 and 30–26, thus handing Riamu his first professional loss. Riamu was deducted a point in the first round for holding Sasaki down.^[9]

Riamu was scheduled to face Aoshi at Krush 122 on February 27, 2021. They were expected to fight at Krush 113 on June 28, 2020, before Riamu withdrew from the bout.^[10] Riamu won the fight by unanimous decision, with all three judges scoring the fight 30–27 in his favor.^[11]

Krush Super Bantamweight tournament

Riamu participated in the 2021 Krush Super Bantamweight tournament, held to crown the new champion. Riamu was scheduled to face the former Krush Bantamweight champion Begin Yoshioka in the tournament quarterfinals, held at Krush 128 on August 21, 2021.^[12] Yoshioka failed to make weight for the fight, which meant Riamu would advance to the tournament semifinals regardless of the fight result. Additionally, Yoshioka entered the fight with a one point deduction and was forced to wear the heavier 10 oz gloves.^[13] Riamu won the fight by unanimous decision.^[14]

Riamu was scheduled to face Yuto Kuroda in the tournament semifinals, held at Krush 130 on October 31, 2021.^[15] He won the fight by unanimous decision, with all three judges awarding him a 30-28 scorecard. Although the first and third rounds were scored as a draw, the knockdown Riamu scored near the end of the second round was enough to earn him the victory. Riamu faced Momotaro Kiyama in the tournament finals. After a slow first round, Riamu upped the pace midway through the second round and dropped Kiyama with a right hook. This was quickly followed by a second knockdown with a left hook, before the referee stopped the fight at the very end of the second round.^[16]

K-1 super bantamweight tournament

During a press conference held on December 21, 2021, it was announced by K-1 that Riamu would participate in the 2022 Super Bantamweight World Grand Prix, which would be held at K-1 World GP 2022 Japan on February 27, 2022.^[17] Riamu faced Ikko in the tournament quarterfinals.^[18] He won the fight by a narrow unanimous decision, with all three judges scoring the bout 30–29 in his favor.^[19] Sera lost his semifinal bout against Akihiro Kaneko by a second-round technical knockout, as he was knocked down twice in the opening round, which resulted in a stoppage victory for Kaneko per the tournament rules.^[20]

Krush title reign

Riamu faced the two-time Rajadamnern Stadium title challenger Mutsuki Ebata at The Match 2022 on June 19, 2022.^[21] He lost the fight by an extra round split decision, with scores of 9–10, 9–10 and 10–9.^[22]

Riamu was expected to make his first Krush Super Bantamweight Championship defense against Lyra Nagasaki in the main event of Krush 143 on November 26, 2022.^[23][24] He withdrew from the fight on November 21, after suffering a metacarpal fracture of his right ring finger.^[25] The title bout was rescheduled for Krush 151, which took place on July 22, 2023.^[26] He won the fight by unanimous decision, with two scorecards of 30–28 and one scorecard of 30–27.^[27]