[5][6] His four-fight win streak earned Kobayashi the chance to face the #6 ranked WBC mini flyweight contender Marco John Rementizo for the vacant WBO Asia Pacific title.
[9][10] Kobayashi defended his WBO Asia Pacific title against undefeated Filipino Roslan Eco on 6 May 2023, scoring a fourth round TKO.
[11] Kobayashi made his maiden title defense against Jake Amparo on 5 August 2023 at the Central Gym in Kobe, Japan.
[12] Kobayashi was knocked down by a right hook early in the opening round and lost the fight by unanimous decision, with scores of 114–112, 114–112 and 117–109.
The championship bout headlined the 11th "Who's Next Dynamic Glove" on U-Next, which took place at the Korakuen Hall in Tokyo, Japan.