Some random Slayer knowledge:
He started the Assassin's Guild to deal with those who considered themselves above the law, and not originally just as weapons to the highest bidder, a purpose that was gradually perverted as time went on. He comes back later to correct this by attempting to disband the guild.
He considers just drinking blood from regular people to be fairly filthy behavior, or so he tells Axl.
He considers Axl one of his best friends, at least as far as humans go. Due to their respective natures, they both seem "immortal", so he finds a kinship with him.
This one is my speculation, but I believe Sharon is the same type of genetic immortal as Raven, due to Slayer's explanation of her abilities. While loneliness, pain, and constant travel have jaded Raven, Sharon likely met Slayer early in her immortality and has been happy to serve as both his eternal companion and "bloodbank" that never runs dry.
The true extend of his fighting ability has never, and likely will never be seen, since he seems to have retired as of his AC+ end.