Ryuusui was born into the Nanami Corp, a conglomerate worth 200 trillion yen they were the biggest name in Marine Shipping. from an early age Ryuusui worked on model ships, ships in bottles using his 100 Million Yen allowance. By the time Ryuusui reached middle school he started captaining his own ship where he sailed around the whole world.

When Ryuusui was below the age of 13 he had his allowance reduced to 1 million yen so he used his allowance and his families reputation to invest in the Financial Markets to acquire his own wealth where was able to buy his own expensive clothes, hire his own personal trainer where he was able to win a Dodgeball Tournament. Ryuusui also liked racing video games where he tried to play a Virtual Reality racing game at a Virtual Reality Center he was refused to try it due to being younger then 13. Due to that he then had a race track built for his personal use.

(Source: Dr. Stone Wiki)