You can’t just go around murdering people. There are RULES, Nimona.
-- Ballister (Chapter 2 paragraph 13)

Importance: Nimona hatches a plot of killing the King and killing Goldenloin. She is stunned to discover that Goldenloin and Ballister used to be best friends, until Goldenloin seemingly betrayed Ballister after a lost joust by tearing off Ballister’s arm with a firearm. He explains to her that his villainy isn’t about winning, but proving a point, and that his rules – such as not murdering people – are his own.

She grows on you.
-- Ballister (Chapter 3 paragraph 12)

Importance: Confronted by Goldenloin at the lab, Goldenloin is interested to see that Ballister has a new sidekick. Goldenloin calls her spunky, while Ballister explains that she has grown on him. This reveals that Ballister has come to appreciate Nimona, and to value her work in villainy.

Killing someone solves nothing, Nimona. It’s vulgar, it’s messy – if you’re going to kill someone...
-- Ballister (Chapter 4 paragraph 15)