Akira only briefly appears in the end, yet the entire movie centers around him.
Akira, as a young child, was recruited by the government for their secret parapsychology project, and was assigned the number 28. The project forced Akira to develop powerful, psychic abilities. During this time, he met and befriended other children involved with the project, notably Kiyoko, Masaru, and Takashi (later known collectively as the Espers.
Akira's powers grew rapidly, and grew beyond even his control. On July 16th, 1988 they destroyed the original Tokyo in what appeared to the rest of the world as a nuclear detonation, starting off a chain of events which quickly led to World War III. Akira was retrieved by the scientists, and was dissected to discover the cause of his sudden psychic growth. The scientists were unable to solve this problem, and the remains were sealed in cannisters and cryogenically frozen underground, sealed away as to prevent any chance of problems in the future.
The existence of Akira, and other remnants of the project become a contentious issue within the upper echelons of Japanese politicians. While the project is considered one of the highest of state secrets, periodic leaks have lead to Akira becoming a messianic figure among some cults in Neo-Tokyo. Minister Nezu, seeing this as an opportunity to gain further political power, manipulates a dissident group, known as the Resistance, to free Akira, to further his own ends.
Tetsuo Shima, upon hearing of the existence of Akira, rampages through Neo-Tokyo in a quest to find him, and learn how to increase his power. Tetsuo finds and destroys Akira's container, and ultimately discovers the truth. Later, when Tetsuo's powers grow beyond his control, causing his body to swell into a grotesque mass, Akira is summoned by the Espers. He appears not as a ordinary human being, but as a spirit-like form who has transcended the limitations of a human body. Akira utilises the power with which he destroyed Tokyo to absorb Tetsuo, and contain him in a new universe.
Tetsuo's friend Shotaro Kaneda was unwittingly caught in the explosion and he experiences both Akira, and Tetsuo's memories before being transported back to Neo-Tokyo by the Espers.
Akira is a young child. Therefore he has a personality akin to a small child his age. He gets along well with the Espers as they were fellow government test subjects, and is shown to have delight in Masaru's display of telekinetic powers. He is willing to help his friends in dealing with a mutated Tetsuo, even if it'll destroy Neo-Tokyo in the process.