They have a sibling vibe right now IMO, (I wouldn’t be opposed to their relationship evolving in the future though, in fact I’d kinda love it, but the characters aren’t ready for that yet). Loved their scenes in “Stable Boy”, he was totally looking at her like a proud big brother when she was putting the clues together. Then in “The Return”, his attitude had evolved into frustration, and then in “The Stranger” he was just plain pissed at her.
It’ll be interesting to see how their relationship changes again once Emma believes who they both are. On the one hand, they have a connection that no one else shares, the two outcast children from FTL who had to grow up alone in our world. But then, once she learns that he left her as a baby, you’d expect she may have some animosity towards him, because if he had have stayed and watched over her like he’d been told to, maybe her life growing up wouldn’t have been quite so miserable.