Edit- AN: As said earlier, this takes place after Robin's rescue at Enies Lobby. To be more specific, it's during the conversation between her and Ao Kiji during the party and I just changed it up. Just to clear up any confusions there may be.
Against all odds, the strawhat pirates not only managed to successfully assault and escape Enies Lobby, but to rescue their nakama Robin as well. As was customary for the crew after every large escapade, the strawhats soon found themselves throwing a celebration that quickly turned into a full-scale party when just about every person in Water 7 joined the festivities. Cheers, toasts, and plenty of singing was occurring as everybody let recent actions slip their memories for the moment to enjoy themselves, everybody save for one person.
Zoro had been contemplating something in his mind for the better half of the party. It popped into his head sometime during the mad dash to save Robin and it was a thought that he had been extremely hesitant to associate with her. Looking around the area, he noticed Robin leaning against a wall alone some distance away, and decided to himself that now was as good a time as any to get it over with and tell her. It bothered him, actually, as he didn't know why he was making such a fuss over it, what he wanted to tell her wasn't that big of a deal in his mind, or at least that's what he kept trying to convince himself of. Finally finding his ground, Zoro gritted his teeth and began to walk over.
It wasn't until he was relatively close to her from the side that he noticed the look on her face, one of complete horror and shock and it almost seemed like she wasn't even paying attention to the scene in front of her. Beginning to become confused, Zoro wondered why she had such a peculiar look when suddenly he recognized the all too familiar voice of the Marine Admiral Ao Kiji. Scowling at himself for his ignorance and stupidity, Zoro wondered how he hadn't noticed the Admiral's presence much earlier and pressed himself against the wall, readying himself for any action that Ao Kiji might take. Senses heightened, Zoro listened in on the conversation to see what was going on.
Doing his best to conceal himself, Zoro started to figure out what the two were talking about. Certain aspects of the conversation confused him, but overall he was able to understand that Robin had the chance to run away from CP9 by leaving him and the rest of the crew behind. The fact that she chose to put her life on the line for everybody only made Zoro more determined to find out what Ao Kiji planned to do.
The conversation between Robin and Ao Kiji was coming to a close and it became clear to Robin that Ao Kiji was going to let her go for now. She exhaled; it seemed fortune was once again on her side and that she was going to be able to remain a strawhat for the time being. At the thought of them, Robin scanned the area to see where each of the crew members was. The cook was preparing another serving of meat for the bottomless pit that was the captain while the sniper was singing on top of a couple of Giants. Relatively close to them was just about the rest of the crew, with the captain being assaulted by the navigator over some squabble as the doctor watched on in fear. A smile beginning to form, she continued to look for the last crewmate, however wherever she looked she couldn't see the swordsman. It was at this point that Robin's thoughts were interrupted by Ao Kiji almost as if he knew what was on her mind.
"Pirate Hunter Zoro," the admiral began as Robin heard a grunt of surprise from a familiar voice, "feel free to un-hitch your sword at any point, I have no intention of attacking any of your crew at the moment. I only dropped by to talk to Nico Robin one more time before I headed off." Looking to her left where the grunt came from, Robin saw the green-haired swordsman and as she looked the two made eye contact. Robin nodded at him, and Zoro's grip loosened over his katana; he eventually stayed his sword, somewhat awkwardly though almost as if he was embarrassed, and brought his posture back from being alert. With that, Ao Kiji finally said his goodbye and left even though Robin had much more she wanted to ask him.
This was the kind of situation in particular that Robin never wanted her crewmates to witness. Always being resourceful and having had to look out for herself, Robin could never accept being in a position of weakness and it only made it worse for her when she was if there were people witnessing it. She took some comfort, however, that it was only the swordsman that came along as she felt a connection with him since they were very similar, whether or not he would like to admit it. Turning to face the swordsman, Robin began to wonder why he was even there at the moment considering how he felt about her being in the crew, that among other questions.
"How much of the conversation did you hear?"
"Not too sure when in the conversation I walked in, but I pretty much caught most of what he was saying to you."
"Why were you over here to begin with? It doesn't fit you to be over here by me while the rest of the crew is celebrating over there."
At this question, Zoro stiffened and Robin thought that she saw a slight tint of red cover his features.
"No reason in particular," Zoro scowled, "I actually just came over here to make sure you weren't going to run away again after we had just rescued you from the marines."
"Is that so? Well I can assure you that I have no intention of leaving this crew for a long time." Robin responded, stressing 'for a long time' to tease the swordsman much to his dismay and to her pleasure.
Beginning to grow frustrated and even more embarrassed than before, Zoro began to walk off, completely forgetting the reason he came over in the first place in his quick effort to get away from the woman who always threw him off guard. He didn't make it very far when Robin asked what Zoro thought was a surprising question.
"Do you still really consider me an outsider of the group?" Robin asked tensely, almost as if she was afraid of the answer.
The sudden change in tone made Zoro consider his answer carefully, perhaps too carefully.
"If what happened to you happened to anyone else in the crew, I would have done the same exact thing." Zoro finally said, instantly regretting the choice of words as he remembered what he wanted to say originally.
"I see..." Robin said emotionlessly, "I assume it's because it was the captain's orders then if I am not mistaken?"
"Well, yeah... I mean, not really... it's just that..." Zoro somewhat stuttered as he tried to piece his thoughts together. Finally at his limit, Zoro put everything that had just happened aside, turned around, and nearly shouted out "Look, the reason I came over here was to let you know that if anything at all has been shown in recent events, it's that you're a part of the crew, Robin. You are one of our nakama and that's all I wanted to say, so there."
Finally free of the burden that had been on his shoulders, Zoro once again turned around to walk away, satisfied with what he had said yet still berating himself for making such a simple thing to say so frustratingly difficult to tell her.
"Zoro," Robin said and as Zoro looked back she finished, "thank you."
He wasn't sure what it was about what she had said, whether it was the fact she thanked him, used his name instead of his role, or that she had the most genuine smile he had ever seen on her face, but it made Zoro feel something towards Robin that he hadn't felt in a long time. Quickly avoiding the thought of what the feeling was, Zoro hurriedly said "Yeah, sure" and walked away as quickly as he could. There was something with that woman, he thought to himself, that he had to watch out for, more now than ever before. Nobody had ever distracted his attention so much nor dulled his senses the way she did, and that made Zoro decide to keep his eye on her yet again, though this time for reasons he wouldn't admit to anyone, not even himself.
As the green haired swordsman walked back to rejoin the rest of the crew, Robin thought over everything that had just happened; the meeting with Ao Kiji, the interruption by Zoro, and the ensuing conversation. It brought another smile to her face as she watched the swordsman off in the distance and said to herself, "My, how interesting."