I wrote my characters voice in first person with another speaker in mind. The secondary speaker doesn't speak at any point, but my character reacts to them through his speech and thus we learn more about him and his voice.

For example

You want someone dead? That’s fine by me, as long as it’s not a kid. I don’t kill kids and I don’t torture people, so don’t even ask. You think a merc with morals is funny? Laugh all you want.