The American Admiral is the head of a naval fleet sent by the American government to investigate the explosion in Neo-Tokyo. He is aware of Akira and the project, which the Americans didn't take seriously until the second psychic detonation of Akira, which makes him the Americans worried about him and the project. For that purpose he gathers scientists around the world in order to study all of this. They all come together in Project Juvenile A.
After he finds out about Akira's and Tetsuo's power he intends to destroy them ignoring the advice of Project Juvenile A of not acting that way because of the tremendous power of Tetsuo and Akira. This becomes an obsession when he sees firsthand the power of Tetsuo, who came to visit the team of scientist and how it changes his body, which he can't handle. This obsession leads him to finally use FLOYD to destroy them, even if it means destroying what's left of Neo-Tokyo and its inhabitants.
Tetsuo stops him by taking control of FLOYD and throw it into the aircraft carrier, where the Admiral was, killing him.