Number 7

Shichiro Oshima wakes up from the artificial hibernation 100 years in the future, sleeping though the wrath of nuclear war.
He wakes up to find Japan to be a primeval jungle, with little to no remnants of Earth's past. After being rescued,
Shichiro learns that the Earth is being under attack from alien invaders from all reaches of the universe.
So he decides to become a member of the Earth Defense Force under codename Number 7 to fight for the liberation of Earth.