The Forever Hero is a trilogy of science fiction novels by American writer L. E. Modesitt, Jr.
[1] Like most of his early work, the books are characterised by heavy usage of onomatopoeia, and a tendency to describe the characters by their physical appearance rather than name.
In this series, Earth has been destroyed almost totally by overpopulation and the ensuing ecological collapse.
The protagonist, Gerswin, is a primitive, a 'devilkid' who is (inadvertently) captured by the interstellar government and trained in the military.
Unique genetic mutations, and the effects of hundreds of years of Natural selection in an exceptionally harsh environment have left Gerswin with exceptional reflexes, enormous physical stamina, as well as, at least initially, a superb memory and incisive intelligence.