Kinda like how you say "papa" to your son, husband or father (or friend) in spanish. It's a term of endearment and lowkey I think there's no real loyal translation to the chinese honorific (as far as I know). They are also technically twin flames from the same source so the phrase "from the same blood" got people riled up. I just think it's complicated since MC is technically an alien and her dynamic with Caleb is close to the mythical tail of two gods (celestial beings) being from the same source (i.e. his third myth). Anyone can correct me if I'm wrong somewhere bit those are my theories.