I stumbled here through Hot Network Questions, so way out of my field here...

But if the narrator is also a character (maybe one that's not introduced till later), make sure to include *personal/opinionated* phrases like 

 - `If you ask me, they should blah blah`
 - `Personally, I believe that blah blah`

And also address the audience 

 - `Don't you think it should be blah blah? Never mind you, I say it should be.`

Later on, the character (who voiced introductory narration), can say those exact things out loud to reinforce it was **their** opinion during narration. 

Rick Riordan's "The Trials of Apollo" is introduced and narrated through the titular Apollo who is also a character, and an arrogant prick. However he develops into a better person over time, and the tone of the narration matches his development. While reading that book, never once did I think it was the author that was an arrogant prick, even early on.