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."
The distinction here being that a neutral narrator (aka author) would not state personal opinion so openly. 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.