I read it in the year 1997 - most likely published 10-20 years earlier than that. Things that I recall:
Cover art had this man-like being engulfed in (wings of?) fire, protecting a girl
Plot deals with a rebel group (?) planning to upstage this master captain (John?) who has an illegitimate daughter he sired on Earth. The girl has not idea she is the heir to this (later-become-vicious) person
There is a love story intersected with an assassin out to kill the father/find (and kill the girl) - I forget which - he belongs to a breed(!) of assassins who are face-changers (and possibly take on the face of the person they kill - or maybe that's just a "faceless man" confabulated memory from GOT
The book ends with this fantastic sword fight between the assassin and the King/Captain/villain father who was otherwise trying to attack his daughter
Also, they exclusively refer to things called "ships" that I later realised were space-ships!
Some planets were referenced, including Mars (I recall that's where the assassin was from - or all the assassins trained there). The final battle took place on (near) one of the Suns of another planet.
It may have included some first-person narration from the girl's point-of-view. I distinctly remember the assassin saying a corny line on how "killing is the second-most oldest profession in the world" and then the girl asks "what's the first", realises he meant prostitution* and blushes.
- I also think she was the daughter of a prostitute.