Ruka is the kind of flash to the past that Duo is both most wary of and most kindred to. It's only really her and Stark that could have anything approaching dirt on him, so he just naturally has to have some mental distance when it comes to dealing with her. Despite that, there's also a kind of kinship he feels to those who he suffered alongside there, and who all know what each other have gone through. The fact that she had been friends with some of his pals and knows their fate, and is capable of reminiscing with him about them only strengthens that. Shared trauma is a powerful thing, and especially so with him (and his cast, it's basically the bonding glue of the Wingboys).

He can definitely see the toll the City has taken on her and he respects her veteran status of information and experience over him, even if he's shitty at showing it. He would not give her information to anyone without careful thought and reasoning, and he also values her good graces a lot. He can tell she's still willing deep-down to be someone that wants to save people from suffering her own fate and that's a quality amongst him and his that he holds in high regard. She's high in his list of important people in this world, not just from a personal but a practical standpoint.