All I remember is a guy who is dying from some kind of radiation, and is trying to find and punish the murderer of his race. He is the last of his kind, and has extreme fighting training, and is one of the best from his race. His race was made up of warriors, and could fight and beat nearly any foe. I think the protagonist was called Keith, or Kyle. He has a ship, and some battle equipment, and, if I remember correctly, a pet of some sort. The enemy was an organization called Darkwing or something similar. I can't remember much more, but I liked the books (it was a series) and really want to find it again!
This is the Last Legionary series.
The protagonist is Keill Randor. The book Galactic Warlord covers the events you're talking about.
[Keill's] home planet was destroyed by a malevolent entity known as 'The Warlord' whose sole aim is to spread war through the galaxy, enabling him to rule over the remains that survive. The Warlord moved against the Legions since he perceived them as the greatest threat to his plan.
The enemy you're thinking of was in fact "a shadowy force known as the Deathwing".
His sidekick is Glr, "a telepathic avian life form from another galaxy."
He is saved from his lethal radiation poisoning by "The Overseers," who "replaced his entire skeleton - using an unbreakable alloy". This plays into the conclusion of this book, where:
The Deathwing attempt to kill or disable Keill by binding him and then inducing convulsions strong enough to break his back. They believe they succeeded; however, his bones are unbreakable and they are deceived into thinking his back is broken and leave him unattended, etc. etc.
This is in fact a dupe in depth:
- Young-adult science fiction novel series. Kid trained as warrior, all other kid warriors wiped out
- Need help identifying this 'space novel'
- Scifi book from the 80's. The last of a warrior race
That said, I couldn't find them until I remembered and could search knowing the answer 8P. So I'm kind of filling this answer out with keywords in case it helps someone find it in the future.