AppearanceSanji is a tall, slim young man with blond hair, often well-dressed, and keeps his hair brushed over his left eye. Sanji's left eye is so rarely seen that many fans have questioned the assumption that he has one at all. In the Manga it is never seen, but is seen in the anime (episodes 21, 27, 82, 128 and Movie 6 although usually very briefly and most likely due to animation errors).  He almost always wears a black, double-breasted suit with tie and long-sleeved, buttoned shirts of varying colors, with or without pinstripes. His fashion changes at times, like in the Water 7 Arc where he had no suit jacket, but a black vest over an orange, pinstriped shirt. He also wears dress shoes, mostly for his Red Leg martial arts. These shoes are extremely durable, and were designed to increase the power of his attacks, though he can very well attack without them as he did against Kuroobi. Sanji is also a chain-smoker and is rarely seen without a cigarette.
According to Oda, Sanji most resembles a duck, is identified by the color blue, and smells of seafood and cigarette smoke.
Sanji's right eyebrow forms a spiral, and he gets very upset if somebody (usually Zoro) chooses to point it out.
PersonalityLike Zoro, he is more perceptive than he seems, and often tells people what they need to hear instead of what they want. Hence, he is seen as one of the more hardened members of the crew and can often be as scary as Zoro at times. Unlike Zoro, however, he is not afraid to show his emotions when the moment calls for it, and is entirely capable of crying or losing his temper. In fact, a running gag has appeared since Sanji's fight with Jyabura, during which he stated that when he gets angry, he 'heats up'. After this fight, when Sanji gets particularly angry at a villain, he appears to burst into flame. This occurs against both Absalom and Duval, though the latter example appears only in the anime.
He is very amorous, constantly flirting with any attractive woman he sees, earning him the nickname "ero-cook". He has sworn to never let women be insulted or injured while he can still stand, regardless of how attractive (or unattractive) they are. This is often problematic in his fighting as he can't attack female opponents; he would rather die than hurt them (though that doesn't mean he won't resort to blocking their attacks). All of this is apparently the result of a "strict upbringing". Also, while most cannot see it, he has a deep respect towards his mentor, Zeff, who selflessly gave all his food to the young Sanji back when they were stranded. As a result, during his time on the Baratie, he exhibits little to no hesitation in beating customers who spoke ill of Zeff's former status as a pirate.
Sanji considers himself handsome and charming, often comparing himself to a fairytale prince. So far, he has found nobody who agrees with him, and tends to be brought sharply back to earth by the more cynical people around him (such as being called the "prince of Retardia" by Zoro prompting Sanji to attack him).
Sanji also likes to swear a lot and is overall a bit of a trash talker, insulting his opponents while fighting - especially if they insult him or his friends first.
One of Sanji's most prominent characteristics is a strong respect towards food and preparation of food in the kitchen; since food and sustenance play a crucial role in maintaining life, he strongly believes that a chef's work must be treated as though it were sacred. Prior to being stranded on the rock with Zeff nine years ago, Sanji thought food was dispensable, and tossed out any that has gone bad. In fact, he was disgusted by how his fellow chefs would eat leftovers from customers. However, after being starved at the rock, he came to regret his maltreatment to food. As a result, he has a strong tendency to become hostile towards those who waste food, or regard it in any way that is less than the way he does. Thus, he has a habit of beating up anyone who steals food from the fridge (mainly Luffy). This is precisely why Sanji, in his first appearance, proceeded to make quick work of Fullbody.
CrewSanji respects Luffy as a captain, but often gets angry at him for eating too much and showing disrespect for the aesthetics of cooking; he also dislikes Luffy imitating him. However, he does care for Luffy as he is shown dragging his captain out of the water more often than any of the other crewmembers. He is constantly in physical and verbal fights with Zoro (who he refers to as 'marimo' (moss ball) because of his green hair) over their differences. Regardless, the two clearly respect each other for their fighting skills and they have developed a bit of a rivalry, almost like that of two brothers. However, though he spends a lot of time arguing or competing with Zoro, Sanji isn't beneath admitting Zoro is right during those rare occasions where Zoro has seen the bigger picture then everyone else (such as Usopp's rejoining).
He is rather protective of the female crewmembers and Nami likes to take advantage of him, exploiting his extreme servility. He's also good friends with Usopp; Sanji understands him quite well. Despite this, he occasionally beats him and Luffy up when they do something immature, like eat too much food or goof off in serious situations. Sanji originally wanted to cook Chopper, and continues to refer to him as their "emergency food supply" to tease him. However, he does consider Chopper a friend, and will give him or Usopp advice when they need it. In short, he cares as much for the crew as any Straw Hat should, even going as far as trying to sacrifice his own life to Bartholomew Kuma for the sake of his crewmembers despite mentioning that he meant to do so for the female members.
FriendsSanji had a habit of getting on everyone's nerves while at the Baratie. However, he parted with everyone on good terms despite this and the entire cooking staff was upset by his departure. They have kept an eye on him ever since and used him to promote their restaurant when his bounty was released.
Despite being enemies, Gin also considered Sanji as a friend because he showed him the only act of kindness he ever experienced. Gin was unable to bring himself to finish Sanji off and left on the vow that the pair would try and meet again on the Grand Line if it was ever possible.
Duval initially hated Sanji for resembling his wanted poster. He was wounded in the back by someone who mistook him for Sanji. However after a defeat during a fight with him, Duval became friends with Sanji because Sanji reshaped his face to make him look handsome. However, Duval's love for his new handsome face gets on Sanji's nerves.
EnemiesSanji supposedly has many people who bear grudges against him from his time at the Baratie. Essentially, Fullbody has a personal vendetta against him for what happened at the Baratie. However, this has never been taken seriously. Like all the Straw Hats, he is wanted by the Marines.
Abilities and PowersSanji is considered to be one of the crew's top three fighters (despite not having one of the top three bounties, which Robin takes). Sanji has mastered an unnamed martial art that the pirate Zeff apparently taught him. All of his maneuvers come from his legs and feet. His legs and feet are extremely powerful, able to smash boulders and defeat creatures much larger than himself (like Bananawani and Momoo). His legs also imbue him with superior speed and agility, able to leap and run much farther than most humans. He has stated that the reason he fights with his feet is that he doesn't want to risk damaging his hands as he needs them to cook, though handstands aren't prohibited. This fictional martial art seemingly takes some inspiration from the Brazilian martial art Capoeira and the Korean martial art Tae Kwon Do and may have took some from the Chinese art Tan Tui and bears an almost identical line, in name and in style, to the extremely powerful and dangerous (to both user and opponent) style Blood Leg. Inspirations may also point to a French fighting style now known as Savate. Originally developed by French sailors, Savate employs similar techniques that utilize the hands to support the body and give added power to kicks, and is especially useful on a swaying ship. And the French law at this time considered hands as lethal weapons, which could lead to death if hands were used to kill someone. Possibly for this reason, Sanji's main opponents throughout the series so far were all martial artists with their own unique type of kenpo (such as Bon Clays "Okama Kenpo", Wanze's "Ramen Kenpo" and Jyabura's "Tekkai Kenpo").
As for his actual "attacks", Sanji has a very large number of specific kicks and techniques he uses to fight with. It should be noted that he doesn't call out an attack name whenever he kicks someone or something, but for important events and fights he usually will. Most of his attacks are said in French and are usually cooking/food terms related in some way to the specific portion of the opponent's body that he's targeting or to the nature of the attack itself. In the 4Kids English anime, the attacks are changed to be funny sounding, non-existent food items that have something to do with the target area of the attack.
Common attacks are Muton Shot (Sanji jumps and flips, and kicks with all his might into the foe's mid-region or head) and Anti-Manner Kick Course (One of Sanji's most powerful kicks where he delivers a 180 degree vertical kick upward into a giant foe's torso or back, which is usually used on those he sees as having no manners, e.g. those that attack women; a typical finishing move).
Sanji using Diable Jambe
In the Enies Lobby arc, Sanji reveals a new ability - Diable Jambe, the power to heat his lower legs to a temperature so high that they glow bright red, by spinning rapidly in place. 'Diable' means "devil" in French and it's also a type of dish. 'Jambe' means "leg", but the kanji reads diable's wind. The aforementioned attacks also have the word Diable Jambe preceding it (ex. Diable Jambe: Flambage Shot). The heat of the attack is so high that contact for one second would set a person aflame. Sanji used this ability during his fight with Jyabura, in which his attacks weren't so effective due to his active usage of Tekkai. With the heat of his new ability, Sanji overcame his inability to smash through Tekkai by burning through it instead. Also revealed during the Crew's battle against the Kuma Clones, it appears that Diable Jambe puts extreme stress upon Sanji's legs. After several uses of the technique, Sanji himself comments that if he were to use the attack one more time, his legs would snap. It is unclear at this point in time whether it is Diable Jambe itself that puts stress on his legs, or the strain of kicking the extremely durable Pacifista cyborgs repeatedly. 
Although Sanji fights with his legs, his arms are nearly as strong, enabling him to use an array of handstands, making him more versatile in certain situations. He is also able to use one-handed handstands. During the Skypeia Arc, while riding the Going Merry, he lifted a large mallet that must have weighted several tons before using it to slam an Impact Dial that had been set on top of a barrel; needless to say, the impact would have busted a hole on the Going Merry's deck had it not been absorbed by the Dial. His upper body is also superhumanly durable, having been known to take head-on attacks like Kuroobi's fists and half of Jyabura's Jusshigan right before counter-attacking them with his kick. His tolerance against fire is also astonishing, which would make sense since he is a cook. This would explain why he is able to utilize Diable Jambe without showing any signs of getting burned himself while the opponent gets incinerated at the receiving end. However, Oda has said that it doesn't damage him because while the Diable Jambe burns hot, Sanji's heart burns hotter.
Though he's one of the physical powerhouses of the crew, Sanji can also be a brilliant tactician if need be. This was particularly evident in the Baroque Works saga, where he was responsible for both disguising himself as Mr. 3 and fooling Crocodile into believing that the Straw Hats were dead on Little Garden and posing as "Mr. Prince" to get the Straw Hats out of Rain Dinners at Arabasta. His skills in this area showed again during the Enies Lobby arc, where he first maneuvered Sogeking onto the roof, where his sniping could be used to full effect. Then appeared again when he shut the Gate of Justice, giving the crew the opportunity to escape the Buster Call.
Sanji also had a fondness for reading books when he was younger and knows of many legends, albeit they are mostly "fairy-tale" like in nature opposed to Robin's "historic" recollection of legends. He had already heard of the legendary ingredient the Blue-Finned elephant Tuna before he saw it. He had also been familiar with the tale of Montblanc Norland through the book King of Liars, Norland. Through a book on Devil Fruits he learnt of the Suke Suke no Mi, although since he had interest in only that particular fruit what else he knows on Devil Fruits is unclear.
Last but not least, Sanji is an expert cook, knowing and capable of preparing almost any kind of food with a strong sense to the aesthetics of cooking. He cooks with amazing speed and dexterity, particularly in his use of knives. Though lacking the power and technique of Roronoa Zoro with a bladed weapon, he is able to make very accurate cuts. He rarely uses them in battle, stating that kitchen knives are not meant for that purpose, but makes an exception when Wanze uses food as a weapon, something that he, as a chef, finds unforgivable, and sees a loophole in using his knives as weapons against food.
A comic side-effect of Sanji's kicks is that, when directed at the face, they can make the victim more handsome, as seen with Wanze and Duval.