Léo Brian Kokubo was born in Chiba to a Nigerian father and a Japanese mother, and joined Benfica from Kashiwa Reysol's youth set-up in January 2019. He had never played senior football for another club.

A tall goalkeeper with a long reach, he progressed through Benfica's under-19 and under-23 sides into the B team.

In 2023/24 he was registered with the first-team squad as third-choice goalkeeper, behind Anatoliy Trubin and Samuel Soares, but did not play for the senior side. His football that season came with Benfica B in the second tier. He left the club in 2024 to join Sint-Truiden in Belgium.

He has represented Japan at under-16, under-18 and under-23 level, and has not been capped by the senior national team.