He spent the 2021/22 season on loan with Belgian first division side Royal Union Saint-Gilloise, linking up with the Brussels based side after a break following the Olympics in Tokyo. He made 29 appearances in all competitions for Union, being credited with eight goals and four assists.

Kaoru was part of the squad which missed out on bronze in their final match against Mexico, after a good tournament on home soil. 

Mitoma has represented his country at senior level, making his debut in November 2021. 

Kaoru would quickly establish himself as a popular player for Albion with his exciting displays down the flank, before scoring his first goal in a 3-2 win at Wolves in November 2022.

He would break the record for most Premier League goals scored in a season by a Japanese player, finishing the 2022/23 campaign with seven goals, one more than the previous record held by Shinji Okazaki and Shinji Kagawa. He finished his debut season for Albion with ten goals and eight assists in 41 appearances in all competitions.