Akai Shuuichi is an American of Japanese descent and member of the American Federal Bureau of Investigation, on special assignment in Japan to capture Vermouth. His superior is James Black and his partner is Jodie Starling. He had infiltrated the Black Organization under the name Moroboshi Dai—and the codename "Rye"—as a mole and dated Miyano Akemi, one of their low-ranking members, to feed information to the FBI. In order to facilitate the latter, and later because he had genuinely developed feelings for Akemi, Akai saw himself forced to break up his then on-going relationship with Jodie. Later on, his cover in the Black Organization was accidentally blown by his colleague Andre Camel, and when Akemi was killed, he declared her murderer, Gin (with whom he seems to have had some tensions even before his exposure), his personal enemy.

Akai is a very professional agent. Even in personal matters, he remains cool, even detached, from anyone, but his redeeming value is a great sense of loyalty to anyone he knows. He is a skilled and relentless pursuer and detective, qualities which have labeled him the number one enemy of the Black Organization and earned him the nickname "Silver Bullet." He looks forward to opportunities to hunt the Black Organization down and get revenge for Akemi. He also acts somewhat independently among the FBI and even his boss James Black has trouble getting ahold of him.