Mezzon Ｚ

The word "Mezzon," meaning "middle child," has been turned into the name of an organization that stands in the middle of the world's two major powers and protects peace. Kenji, a curious boy, saves his brother, Mezzon J, a member of the Japanese branch of Mezzon, from a difficult situation and becomes a member of Mezzon. Just as Kenji becomes accustomed to handling the Mezzon uniform with its various secret weapons, his older brother goes missing. In order to rescue him, Mezzon Z heads to the "Island of Superhuman Brains," but...?

(Source: Kodansha, translated)