I read this at local library once, I liked it, but title has always escapes me. It sort of followed the unlikely hero trope. The one thing that I clearly remember were the Viver. These were not aliens, but transhumans. I recall them being warrior culture, with natural body armor.
This could be Spacer: Window of the Mind, by John Maddox Roberts, or the related book Space Angel. Credit where credit is due: the presence of the name "Vivers" makes this easy to find, and others have done it before I. I can't search inside the book, but there's still some background. See this question or this one.