Vyken Dark by Christine Myers

Cyborg Vyken Dark is back on Earth after fighting for eighty years in the Procyon War against the Mesaarkans. He came to help restore order out of chaos in old Chicago USA. Every major city on Earth is in ruins. Most are now run by gangs of thugs ruled by overlords who take everything of value and prey on those who are just trying to survive. Vyken and his four brother cyborgs are all that’s left of their starship crew of three hundred.

He didn’t want the job, but when his old mentor Admiral Carson Gregor asked, Vyken Dark could not refuse. If he could have chosen a man to call father, it would have been Carson Gregor.
Vyken chose the name Dark because the darkness in his soul drove him to kill the enemies of the Federation with a ruthlessness that few natural humans possessed. Sometimes that darkness overwhelmed his humanity and made him weary of his life… Until he came back to Earth and found her.