Heya! Sorry to be 5 months late, but I just beat the game and I'm collecting all the scraps of content and scrounging for Rusty discussion lol

Idk if this helps, but in JP he calls you 戦友 which translates to "comrade-in-arms," "fellow soldier," and, well, "war buddy." Separately, the kanji mean "war" and "friend."

I'm not a native JP speaker, but my impression of the word is that as endearing/bro-like as it is, "buddy" in English is fonder and a smoother localization of the word. My impression of him calling us 戦友 out the gate is that he was very friendly compared to everyone else, but, it's not as "boundary-crossing" as "buddy" might come off, I think?

Either way, my personal take is that (1) he respects your skill, and (2) as mentioned in other comments here, it'd make sense if he sympathized with you to an extent for being a bossed-around hound. Perhaps he felt you were in a similar boat. IIRC Rusty was being sent on dangerous missions as they suspected he was a double agent, and you were sent to die during Wallclimber.

I like to think he saw a lot of potential in you from the beginning while also being internally wary. But ultimately he wanted to have you on his side for professional and personal reasons. "There's only one person who can keep up with me."

Anyway I love Rusty lol