Thought of a few more:

Miro as an NPC Barbarian King. Believes himself chosen by [insert preferable war-type deity here] to be the champion of a city, realm, etc. Comes off as super friendly when you're on his good side, but is quick to temper if he feels you are wasting his time. Does everything in the name of his God. Probably a Path of the Zealot Barbarian. Maybe design him to only be at 14, so the boss encounter is leveraged around the Rage Beyond Death feature, which states that Miro/the Barbarian, even upon hitting zero hit points, stays "alive" until his rage runs out in 10 combat rounds. Make the battle a survival-type affair.

Turn Lance Archer into a literal Murderhawk. Large intelligent bird demon that can only be calmed/satiated by death. Because "Everybody dies." You could build a culture living under the Murderhawk's talon that is forced to make sacrifices from their own or passersby (like the party) in order to live peacefully. Huge plot hook to work with there.

Jurassic Express, formerly known as A Boy and His Dinosaur. A boy and his dinosaur.