In the film, not so much a boy as an entity. The government had him destroyed for not
having the technology or undestanding to monitor his condition, but kept his remains
buried in an underground vault. Believed to have caused the explosion on Tokyo
which started World War III, his psychic potential is the highest monitored by scientists.
His 'essence' lives on through the other espers who were at the same lab and they
vaguely communicate via telepathy. Kiyoko describes him (telepathically through Kay)
as energy thought to be the ultimate form of human evolution, too dangerous for modern
man to possess. In the past, the government tried to harness that power, they failed
and the destruction of Tokyo was inevitable.
In the manga Akira is a living child, who still holds great power. He is worshipped by
the people who live under his Great Tokyo Empire with Tetsuo as Prime Minister. He
is cared for by Kaori who acts like his sister, as ordered by Tetsuo who wishes to
harness his power.