Sanji wasn't sure what to think about her. She was the most masculine woman he'd ever met, and even went out of her way to trick people outside the crew into thinking she was a man, and ignored all opposition to sleeping in the men's room while on the Merry, claiming she could take care of herself, that no one would dare come after her, come onto her, if they wanted to survive. She was an enigma to him.
She wasn't like Nami. Nami may have taken advantage of him, but he loved her anyways, probably more than any other woman he'd ever encountered, not that he knew that part himself. She may have had a temper to end all and a unique, electrocuting weapon to boot, but she was feminine, and looked it, acted it. She was his favorite, and every time he did something for her, his heart gave a little leap.
Unlike with her. When he did a favor for her, which wasn't often, all he could do was hope he did it right, just hope to see that nod of affirmation that showed she was satisfied.
She wasn't like Robin. Robin was calm, cool, and collected. She always had that knowing smile, and when he helped her, he always felt like he was helping her unravel that great mystery that she was after, just by making things easier for her. Robin reacted to his flirtation with a polite smile, but nothing more, which was good, considering she was a good nine years older than him, loath as he was to admit; she was on the run from the government before he was even born, and though he flirted, it wouldn't have felt right to have a woman of that age flirt back. When he talked with Robin about something, he felt like she appreciated his opinion on something she was wondering about, meager though his offering may be.
It wasn't the same with her. She, when she talked to him, kept it short, and not particularly sweet. If he did say something, she would listen before either dismissing it or taking it into consideration; the latter, however, was so rare that it barely mattered. She didn't really talk to anyone on the crew much, other than Luffy. Oh, she may have discussed fighting techniques with Brook once in a while, or pointed out a small nick in the wood to Franky so he could smooth it out, asked Nami when they would reach the next island or Robin what she was reading, but she didn't talk much with anyone; Sanji even somewhat envied Luffy for his ability to get her to talk with him, sometimes for hours on end. The most Sanji could do was ask her what she thought of the food, and even that rarely resulted in more than a simple "Good."
She wasn't like Vivi. Vivi was young, and, when her mind wasn't dwelling on her country's fate, carefree and innocent. She reacted to his flirtation with confusion, which he was somewhat thankful for, since she was three years younger than him, another age difference that he acted like he didn't see, but felt uncomfortable with nonetheless. She was innocent, and thought she could stop the war without bloodshed, that her own life was all she had to risk, and he was happy when he saw such innocence in her, for it was rare in a time with as many pirates as their own.
She was different, very different. She had killed before, before he even met any of the members of the crew, before even going pirate, back when she was still hunting bounties. She had probably killed more than anyone on the crew other than Robin, and maybe Brook; they didn't know enough about Brook's past yet to tell, though Sanji guessed that it still probably wasn't as much as her. Considering her fighting style, she'd probably spilt more blood than either of the others. She was dangerous, and when he saw her in battle, he shuddered at the bloodthirsty smile she wore, at the killing intent she leaked continuously. On a man it would have been bad enough, but on a woman, a smile like that only said evil things, things he didn't like seeing on a woman's pretty face. He shuddered whenever that smile came back to him in frightening clarity, because on a woman, the smile seemed only worse, like a true demon.
She hated his use of honorifics, and he'd long since learned that adding in that cute little w like he did with Nami-swan, Robin-chwan, and Vivi-chwan was asking for a death sentence. She'd insisted that he treat her like he did the male crewmembers, and thus he'd used –kun when speaking with her, if any honorific at all, and though she rolled her eyes, he saw a small glint of satisfaction in her eyes that she was spoken to like a boy, even if she wasn't treated like the ship's male crewmembers.
She didn't often ask him for favors like Nami, but when she did, he worked as hard, if not harder than, as he did for the other girls. The only difference was that he knew this had more riding on it, because, somehow, he craved her approval, and for a very different reason than the other girls.
She scared him.
He didn't like admitting it, but she did. He flinched a bit every time someone asked him to go wake her up for a crew meeting or a meal. He knew she loved her sleep, and waking her up, he was always ready to fend off an attack. He knew that if he answered wrong when she asked why he'd woken her up, he'd have to deal with a grumbly woman for the rest of the afternoon, and be forced to give her more sake than usual out of guilt.
He didn't like admitting that he'd glimpsed the carnage on their way out of Whiskey peak, and had to hold back nausea at it, learning only later that she'd been the one to do it. When he'd seen what she'd done to her ankles at Little Garden, he'd been horrified, but told himself it was a one-time thing, that she just overreacted from being so close to death. When he'd heard that she'd beaten up all of Wapol's soldiers just to grab a coat, he'd shuddered to think that maybe she was as much of a madwoman as he sometimes thought. He may have also been actually happy to learn that she had been wearing both her bandages and a tank top, and wasn't completely bare on top when she'd stolen the coat.
He didn't like admitting that every time she forced him to spar with her, claiming that he was the only other person on the ship with a fighting style and strength worth sparring with, since Robin and Luffy relied on their Devil Fruits (though Luffy could probably provide the workout anyways, but he was Captain), and the others were too weak, he feared that he would come out of it with more than just a couple of bruises and cuts. He didn't like admitting that her grin scared him, and that his fear of getting his throat slit by the girl had been, though it no longer was, the only reason he actually fought her, even though he hated fighting women. Only because she'd insisted that he treat her like a man, that she be treated the same as if she was the person the world thought she was, did he actually force down the nausea that he felt at fighting a girl and actually spar with her, at least in the beginning. He'd later consented to the fighting when he realized that fighting her actually helped him get better, was an actual workout that no one else on the ship was strong enough to provide, and he began to enjoy it. Enjoy the fact that he wouldn't die, but she'd definitely make sure he came close if he held back even the slightest bit.
He didn't like admitting that he was scared whenever she came back from fights. She was always covered in blood, and he had to wonder: if this was her state, and she'd won, then what did her opponents look like? The only person that kept him more worried was Luffy.
He didn't like admitting that one of the scariest things about her was her resilience. He knew that he was considered the third strongest fighter on the ship, sometimes tied with Robin, and he also knew that he was a lot less likely to be gravely injured. Whenever everyone came back from a fight, most of them were hurt at least in some way, no one was unscathed, but it was rare that anyone was hurt to her level, or Luffy's. It scared him when he heard what happened on Thriller Bark. Afterwards, he kept an ever careful eye on her, wondering just how much of a grudge Death must carry against her by now for escaping his clutches so many times.
He didn't like admitting that even her attitude scared him. When Nami was angry, she'd yell and hit him. When Robin was angry, rare though it was, he found himself being ignored completely. Vivi had never gotten seriously angry at him, and the only times he'd seen her angry at anyone, she'd cried. But she…
When she was angry, he saw it mostly in subtle ways. She wouldn't compliment his cooking at all, whereas normally she at least acknowledged it. She fought harder in their spars, a wild look in her eyes, and he flinched when he saw it. She'd wake up without complaint, but would throw him dirty looks afterwards. And, in the one not-so-subtle action, she'd ask a favor.
He hated this favor, not only because it meant he had managed to lastingly irritate her, but also because the favor was designed to mock him. She'd ask him to bring over one of her weights, one of the ones he didn't have the upper body strength to lift easily, and couldn't lift it with his legs alone because he would have to toss them to her, and even the Sunny would have a dent from weights as large as hers. She'd watch him struggle with an unreadable expression on her face, and smirk if he dropped it or set it down. When he was most of the way there, she'd roll her eyes exaggeratedly, walk over, and pick it up with ease, and say the word he'd come to hate more than any other. "Weak."
He knew not to flirt with her: he'd spent too long on the ship with her. Luffy had laughed the first time he'd flirted with Nami, because he'd ignored the other woman, who'd smirked behind her glass. She was surprised afterwards when Luffy told her that Sanji had asked why she hid her gender, and confronted him about it later on. After his reply that it wasn't up to him to remove another's mask, she'd nodded, a serious expression on her face, a gesture he'd learn to cherish, and asked how he'd known. When he'd replied that he always knew when a pretty girl was nearby, and winked, she'd decked him. When he met up with the whole crew at Cocoyashi, she'd stared a bit, as if trying to figure out what to say to her newest crewmember, before shrugging and continuing to yell at Luffy for getting her run over by Sanji's own boat, and, after the Arlong fiasco had finished, worked out a couple of rules with him, the main ones being not to flirt, and to treat her like any of the male crewmembers. He had, after some persuasion and a couple of not-so-subtle threats, agreed.
When he'd found out that she slept in the men's cabin, he'd been horrified. She simply rolled her eyes and said that she'd kill anyone who tried anything, and when it was pointed out that Luffy was strong enough to beat her in a fight, she'd rolled her eyes and said, "He may be strong, but he's a complete moron. He wouldn't try anything anyways." Sanji had finally accepted, and found it awkward to sleep when there was a (snoring) woman in the hammock below him most nights. When she'd finally been forced by Franky and the majority of the crew to move to the women's cabins in the Thousand Sunny, he'd been happy, happy that he'd finally be able to ignore the fact that there was a woman sleeping in the same room as him, and get some sleep on his own. He'd then despaired when, time and again, she'd snuck in in the middle of the night grumbling about Nami being whiny or something of the sort, and told Luffy to budge over and let her sleep in his bed.
This situation is what lead to some confusion on his part. Was she in a relationship with the captain? She'd made no indication of it before. He had watched carefully and finally realized what it was, besides the typical Captain-First Mate relationship. They reminded him of twins that he'd sometimes seen on the Baratie. Not the identical ones; those had always made him feel a bit uncomfortable, creepy as they were, but the fraternal ones. The ones that weren't necessarily noticeable as being close, but if you watched, you saw that they understood each other, and they understood the smallest of movements that spoke volumes to the other twin, but nothing at all to anyone else. She'd managed to form a bond closer than most siblings with the captain, and Sanji found that he enjoyed watching the two interact, because they were closer than anyone else on the crew, and they weren't even lovers. He'd once heard Usopp ask Franky and Chopper what they thought the relationship between the two was, and, against his better judgment, intervened on their behalf, though he'd been teased, with the others saying they'd thought he didn't like her in the same way as his darling Nami-swan and Robin-chwan.
Brook, Sanji had enjoyed seeing, was also very scared of her. Sanji enjoyed laughing whenever he saw Brook ask Nami and Robin about whether he could see their panties, and then steer clear of her, his empty eye sockets watching her glare as he edged away from the girl, hoping not to be hit by one of her swords, and attempting to placate her by asking her about what she thought of his new technique, admitting in the process that the woman, though not even a quarter of his age, was more skilled than he. She smirked whenever this happened, and eventually Sanji had, just by watching them, learned how to see the flaws in a swordsman's technique, something he assumed would be very helpful in coming battles. She had noticed him watching, and once asked if he wanted to join in and help her and Brook in finding out how to make their techniques better. Since then, he'd often joined in, and even heard her laugh a couple times, asking them what they thought of her working with a pair of perverts rather than on her own, and which option was really the lesser of two evils.
Eventually, Sanji had come to terms with the woman, and figured out an uneasy balance between them. The truce wasn't exactly perfect, but it worked, and they'd managed to get through the journey without any major blowouts so far. She scared him, intrigued him, and he'd come to view her as a somewhat violent, slightly older sister. He'd long since learned that she was not a romantic option, no, that was reserved for Nami-swan, if she ever came around, and he'd learned that treating her like he did other women was nothing less than a death sentence. He'd learned about her, learned to live on the same ship as a woman who had killed many long before even meeting him, and had killed often since. He had a brotherly affection for her, and had learned to live with the Demon of East Blue by his and his nakama's side.
He learned to live with the woman known as Roronoa Zoro.