So this essay has been a long time in coming. Read up on Rinko and Sano. With random junk about Robert in there somewhere. Be warned for it being long and rambley and possibly incoherent.
The biggest [and by biggest I mean most visible] aspect of Sano and Rinko's relationship is definitely the bickering between the two. They just clash. One of Sano's defining personality traits is that he thinks and acts very quickly most of the time - especially in fights. He doesn't always stop to think through what he's doing. If it's fun or seems like it'll work at all he doesn't worry about it until after the fact. One of Rinko's traits is the exact opposite -- thinking and acting slowly, in a way. It's not so much that she thinks slowly as it is that she just thinks through each and every possibility to choose the best one for the situation. And this drives Sano nuts. He's a get up and go do it now now now kind of person, and having to wait on her to decide what to do is like nails on a chalkboard to his add self. He doesn't like sitting around and waiting for her to decide what to do, but at the same time she's his teammate and friend so he has to. But damned if he's gonna like it.
To a somewhat lesser extent, it bothers him because it's just not safe in his opinion to hesitate so much about some things. In one episode [31, I think] Sano and Rinko are attacked by heavenly fruit. Rinko stands around and thinks for several minutes while an orange chews on Sano's head and he eventually has to knock it off himself. When it attacks Rinko next, he reacts by smashing it with a steel bar. Sure it hurts Rinko too, but the point as far as he's concerned is that he took care of the problem and, y'know, kept her from getting her head eaten. And he just can't understand why she'd want to take so long to think about such an obvious and immediate problem - and if she'd wait that long over one of her friends then how long will she wait over herself. So it's all very, very frustrating worriesome. D: It's not quite as much of an issue now though -- Rinko and Sano have resolved their differences as of the point I've pulled Sano from. They work together and play their strengths off of each other to make up for their weaknesses. Sano can recognize the strong points of Rinko's strategy, even if he does find it ultimately annoying. And they still bicker about it because it's annoying outside of battle anyway.
Even before that though, they were probably friends anyway. They spent a month [or two? gotta check] together in a hospital recovering from injuries after the Dogura Mansion fights -- same room. For two months. They definitely had to do some talking during that time, and they both seemed to get along decently during the episodes with glimpses of them in the hospital. And before that they both naturally worked together to fight against Robert during the Dogura episodes. There is a natural sense of teamwork between them.
And then there was camp! Heavy summarizing here - check Sano's memories for threads with Rinko. There was much of the expected bickering and arguing, but one of the
first they had was an argument about whether Sano should act alone in a fight or not. Sano was very adamant about working alone if the situation didn't offer any other options and was annoyed that Rinko wouldn't agree, but he could see the reason in what she was saying even if she didn't agree. And it's nice to be worried about like that. Also I love that thread it's a good example of the way their camp relationship worked. "snarksnarkworrysnark>(" Ignore the broken icons.
Overall they got along fairly well - going to each other with various problems around camp like the issues with Robert and Anon, or figuring out how to switch Robert and Mori back; comforting each other over camp afflictions like Sano turning into a tasmanian devil or Rinko switching with Mal. Close friends. It really hit Sano hard when Rinko left camp - it hadn't occurred to him that they might leave camp separately, and she'd been acting oddly just days before which only made him wonder even more what had happened to her. Looots of worry before he was finally able to convince himself she must've left safely.
Jumping around more! Rinko canonically loves Robert. Sano is canonically aware of this. And he puts aside any lingering resentment of Robert to take time to assure Rinko that they'll save Robert and everything will be okay before the last round. He has plenty of reason to dislike Robert, what with the mess with the Ten and his candidate, but he puts it all aside in favor of cheering Rinko up and working to keep her happy. And in camp this extends to actively trying to be nice to Robert, though at this point in time part of it is from genuinely wanting to help him out. A happy Robert = happy Rinko in Sano's mind. And a happy Rinko is very important to Sano.
To be simple about it, Rinko is Sano's teammate and possibly his closest friend among the members of the team. She drives him up the wall at times, but he's aware of the good intentions behind her nagging and is thankful for them. He relies on her as a friend and teammate - she provides support to him when he's down, and he does the same for her. He knows he can rely on her in a fight and trusts her to have his back, and he'll do the same for her without thinking.
And as a final ooc note: I do play Sano as having a slight crush on her at this point, but I don't expect it to go anywhere should a Rinko be successfully apped. That kind of thing depends heavily on how they interact in camp and stuff. I'm not pushing for a relationship between the two characters. D: Not against it at all, but not, y'know, actively for it. Just saying. Riiiiiiinko come snark with me. ♥