First, given how unwieldy the phrase "Demosthenes & Thalassia" is, I hereby dub them D-Thal.
I've commented here & there that an important influence on D-Thal's relationship is that of Elric & Stormbringer: the weapon and wielder between whom the lines of control are blurred. Of course, Stormbringer had a personality only in a loose sense, and didn't speak except for the one famous line. A more recognizable model for D-Thal which recently popped back into mind is Jinto and Lafiel from the anime Crest of the Stars. I'm not big into anime, but this is my favorite by far. I'll leave to you to look up the premise, if you're interested, but readers of Athenian Steel will definitely find something familiar about the relationship between a backward but open-minded ground-dweller and a star pilot from a genetically superior race of amoral interstellar conquerors. The space opera plot is good, but what makes Crest shine are the little moments shared by Jinto and Lafiel as they get closer and are forced to depend on each other. She calls him "Baka," Japanese for "idiot" but her use of the term gets progressively more affectionate.
Crest is based on novels by Hiroyuki Morioka, which I have read in translation. His Abh culture is a fascinating creation. The anime seems to be a very faithful adaptation, and honestly I'd recommend watching over reading. The video is not extensively available in the US, but you can get used, single-volume DVD's for cheap. Here's the first one. Just be sure to watch it in Japanese with subtitles, not the English dub! The dialogue is much better.
Sometime I will explain how Gaeira from The Path of Ravens is based on EVA from Wall-E.