I think I found what their relationship should be. There are many clues for it. I think Ao Kiji is the father of Robin. Why you ask? 1. First look at the appearances. If you compare Robin with her mother, you will immediatly notice 1 big difference: their hair color. It implies that robin's father must have black hair. Ok, not much of a clue, but this got me thinking, so on to point 2. 2. Now the meat. The very first time Ao Kiji met robin, he actually let her go (after he killed saulo). He explicitly told her to live a normal live and not to do anything stupid. But why would a stranger do that and risk his own life for that? Ao Kiji even sabotaged the helm of Spandine's ship, with ice! The only reason I can think of is because he wants to protect her for some reason. 3. The second time they met is when Ao Kiji almost killed robin. But why would he embrace her in the end if he just needs to kill her? That's just awkward if he doesn't have any feelings for her. I doubt he fell in love with robin while she was just a little girl, so it must be a bloodrelated relationship. 4. After the last chapter, we see that Ao Kiji was actually watching everything but didn't do anything to help the buster call out. He could easily have destroyed or stopped the strawhats as he can freeze the sea. So why didn't he do that? My guess is that he wanted to test Luffy's strength and wanted to see whether Luffy can protect his daughter if he let robin be with them or not. "Test? He almost killed luffy!" you say? So, point 5. 5. Remember Ao Kiji said that he wasn't on duty when the strawhats met him on that island, so he doesn't need to kill them? That's because he still doesn't want to kill robin. But why did he change his mind? He changed his mind after he saw how good of a doctor chopper is. After he knew chopper could even heal frozen body's (Ao Kiji told him that robin is still alive and can be saved if he was quick), he decided to demonstrate his power to encourage luffy to become stronger so he can protect robin. He purposely didn't kill anyone (of course) and certainly didn't want to even attack chopper as he had to save the injured. He had to test luffy's strength as I said before, so he set everything up to do so (by giving spandam the golden mushimushi for the buster call). But I bet even he didn't think the strawhats could have survived enies lobby. So far, everything Ao Kiji has done has helped robin or the strawhats in some way, so I can really see him becoming a strong ally in the future. Also, it should be obvious he has caring feelings for robin, so that's why I think that Ao Kiji has to be Robin's father. Just imagine how a kid of robin's mom and Ao Kiji would look like.