Roronoa Zoro, is a pirate and former bounty hunter. He is the first person to join Luffy's crew, the Straw Hat Pirates, and is a master swordsman. Despite no clear role being given to him, Zoro is sometimes labeled by fans as the second-in-command of the pirate crew (the First Mate). Partially due to the fact that in the series, several captains refer to their first mate as their "Partners" or "Champions" and Luffy refers to Zoro by these earlier on in the series. Zoro is currently one of eleven pirates who have been referred to as "The Eleven Supernovas". He and Killer are the only supernovas who are not captains, they are also probably the only Supernovas not to have a Devil Fruit and they both rely on blade based weapons.
He is almost always seen carrying around his three swords, usually bundled up with his green sash (or haramaki) over his right hip which is favorable so he could easily draw any one of them with his left hand; his right hand is usually seen resting over the swords. This may indicate that he is left-handed or even ambidextrous, for most sword wielding characters in the One Piece Universe wear their swords to the left. On his left ear he wears three earrings and he normally has a bandanna tied around his left arm that he ties around his head when he is fighting seriously. The rest of his clothes vary from time to time, however he usually wears a white shirt, dark green trousers, black shoes and the green haramaki sash around his waist.
His body is covered with scars from his many battles- during the Baratie Arc he lost a duel with his rival, Mihawk, and was left with a massive scar across his torso. In the Little Garden Arc he tried to free himself from a wax prison by cutting off his feet. Although he did not entirely succeed, he gave himself a scar around each ankle. These scars are marked by black stitches
His other remarkable feature is his green hair, of which Sanji often teases him about, calling him 'marimo' (moss-ball, moss-head in the Viz Manga and FUNimation dub).
According to Oda, Zoro resembles a shark. Oda uses the color green to represent Zoro, and he smells of steel.
Zoro usually maintains a stern, serious and distanced personality, but often loses his temper in a goofy and exaggerated comic style. He has a strong sense of reason when it comes to actual leadership of the crew and will often point out the seriousness of situations (such as Usopp's leaving the crew). And while technically not a samurai, Zoro maintains Bushido to a certain degree (i.e. he consistently refuses to attack an adversary whose back is turned on him, and neither does he tolerate getting assaulted from behind himself except when his crew mate is really in dire situation of dying of from an enemy threat, such as Kuma trying to take Luffy away, Zoro silently attacked him with Shishi Sonson) and is constantly mistaken for one. Whether he agrees with them or not, he will stand by Luffy's decisions, stepping in only to voice an opinion when the others have failed to see the true reality of a situation (again, such as Usopp rejoining the crew). He, like Luffy, has a lousy sense of direction (possibly even worse than Luffy's, since he usually assumes the others have gotten lost, and not the other way around). He is teased often because of it.
When the crew sails from one island to another, he normally either trains with weights or just sleeps and is normally awakened in a rather violent way by either Nami, Sanji and (accidentally) Luffy. His only apparent job on-board the Going Merry was using his enormous strength to haul up the anchor. Aboard the Thousand Sunny, however, he has been shown to seemingly spend most his time in the Crow's Nest (which is also a gym), so it is possible that he was designated as the ship's lookout. He also likes sake, almost to the degree that Luffy likes meat, but like Nami, he mostly never gets drunk due to his inhumanly-high tolerance for alcohol. He is sometimes prone to using serious methods for solving simple problems such as separating him and Usopp from cuffs by suggesting to cutting off a hand, and cutting off his feet to get himself unstuck from some wax.
Zoro can also be very vigorous and won't back down when he is determined to achieve a goal or defeat an enemy. The risks that Zoro will often take to reach his dream of becoming the best swordsman often leaves others wondering if he is crazy. As far as Zoro is concerned though, only he has the right to call himself "Crazy".  However, he is not one who stubbornly refuses utter defeat, especially if it all seems so inevitable. When Zoro has the intention to fight seriously, he normally ties his black bandanna (which is usually tied around his left upper arm) over his head. Furthermore, like any great warrior, Zoro is one who lives for the thrill of combat; in fact, he has a habit of showing a joyful smile whenever he faces a particularly skilled opponent, which often unnerves both ally and foe alike.
Also, Zoro can provide important and logical decisions at critical times by evaluating the situation, as he did when the crew wanted to bring back Usopp in the Enies Lobby arc. Zoro is the one who mentioned that Usopp is at fault for leaving the crew on his own accord, and Luffy shouldn't take him back unless he apologizes for his mistake.
He also is one of the first Straw Hat members to suggest killing someone or threatening to kill someone.
Zoro is considered to be one of the Straw Hat crew's top three fighters; so much that people occasionally mistake him as the captain of the Straw Hat Crew after witnessing his fighting skills in battle. Mainly, while Luffy always fights the most powerful enemy, Zoro always fights the second strongest. Even without his swords, he has monstrous strength. He is able to toss an entire building at an opponent and even able to deflect Oz's punch singlehandedly. He can use some sword techniques without his swords. Zoro has shown to pack powerful punches. He even developed the ability to pump his arms up with pure muscle.
Zoro is unarguably one of the strongest characters revealed thus far. While most super powered pirates and villains gain power from eating one of the Devil Fruits, Zoro has gained his powers through hard work and training. Zoro has enough power in his lower body to allow him to keep up at a running pace with Sanji, who is a master of the Red Leg style. He can also move at near-inconceivable speeds and jump astonishing heights. He can even dodge bullets with ease. He has been known to effectively knock out much larger creatures with unarmed strikes. Zoro also defeats opponents with massive punches and kicks. In Enies Lobby Arc, the Franky Family and Galley-La carpenters noted twice that Zoro was strong enough to be the captain of the group.
To match his strength, Zoro also has ridiculous amounts of stamina and endurance. Some of the things Zoro has endured include Mr.1's two buzzsaws jamming into his chest during the Arabasta arc, Arlong's saw-like nose stabbing into his face, Mihawk's Black Sword slashing his chest, CP9's Kaku's many Rankyaku attacks, and Kuma's Ursa Shock. Zoro managed to stay conscious while everyone else was incapacitated. Zoro then convinced the Shichibukai to transfer all of Luffy's pain and fatigue into him. Kuma was confident Zoro wouldn't live. However, the swordsman was seen standing blood-drenched at the end of the chapter/episode, claiming that nothing happened.
During almost all of his major battles, Zoro is usually seen covered with blood at the end (Oda has stated that Zoro lost 5 liters of blood fighting Hachi and Arlong at Arlong Park [It should also be noted that the human body holds about 8 liters of blood]). Zoro's agility, speed, and overall reflexes are also extraordinary. In numerous occasions, Zoro has shown being able to evade (or block) high-speed attacks and projectiles. These high-speed attacks and projectiles move so fast that it would normally be impossible for an average human to even notice them, much less evade them.
Though he is very perceptive during battle (as well as out), his sense of direction is extremely poor, to the point of getting lost on a straight path. This is often the result of Zoro not understanding directions. In the Arabasta Arc, he mistakes going north for going up. In the Drum Island Arc, he believes that he can keep the ship on course by heading toward a large cloud (which can move and change shape). He is unaware of his lack of direction. Everytime he gets lost, he mutters "the others are lost again". This is further made fun of with Jigoro in Thriller Bark. Jigoro had Zoro's shadow implanted in him. While the Zombie Generals were heading to help, he goes in the opposite direction. However, Zoro has other strong senses and is often one of the first members of the crew to hear an enemy approaching or sense danger.
Unlike some of the members of the crew, Zoro does not shrink from the idea of using deadly force against dangerous enemies.
Zoro is a master swordsman, being able to use one, two, and three swords in varying attack styles, going from melee-type all the way to artillery. Although he is most comfortable with his Santōryū (literally: three sword style), his skills with a single sword are very good, being able to hold off the Nyaban brothers, having bested several of the lower-ranked members of Arlong's crew (his two other swords were destroyed by Mihawk, leaving him only with the Wado Ichimonji), defeating Mr. 1, and most recently the zombie, Ryuuma. Lacking a third hand for a third sword, Zoro holds his third sword in his mouth. Some of his sword techniques use the air itself to hurt the enemy. Zoro often speaks with his sword held in his mouth, though it's unknown how he manages to do so. The creator of the series, Eiichiro Oda, has said that it is his heart that allows him to speak, though Oda is known for giving joke answers to fans' questions, as well as just making up answers on the spot to avoid leaving a fan's question unanswered.
Since no one else has been seen using anything similar, it can be assumed that Zoro invented not only Santōryū but most, if not all, of his sword techniques. He is even seen perfecting a new technique in his fight with Braham.
If he is fighting weaker enemies (or innocent bystanders that he has no desire injuring in the first place), like the Galley-La employees in Water 7, he uses only the blunt sides of his swords. Although, as Nami pointed out, due to his strength, even that could cause fatal wounds. A long standing problem with Zoro was his inability to cut steel. He managed to instinctively cut steel during some of his encounters before learning how to control this ability during his battle with Daz Bones.
Zoro has shown a new ability called "Asura",which gives him the appearance of having six arms and three heads, like the deity of the same name. The exact nature of this attack is currently unclear, but Kaku suggested it was an illusion created by Zoro's spirit. He cut through Kaku's attack, and was able to deliver the finishing blow. He also may display a kind of haki, as he was able to instill fear into kidnappers at Sabaody Archipelago and because of the mysterious nature of the aforementioned Asura techniques.
Zoro using "Asura",with 6 arms,3 heads and 9 katanas
Though Brook became the crew's 2nd swordsman, the level of mastery and power between the two is significantly different, made obvious when both swordsmen have attempted to take on the same foe. While Brook had been unable to defeat Ryuuma (no matter how hard he fought) while Ryuuma only used a fraction of his power, Zoro was able to defeat him at full power with only two of his swords. At the Sabaody Archipelago, Brook struck a Pacifista with a high speed sword attack, yet failed to do much damage to it and had to be rescued from this result afterwards. However, when Zoro attacked the same Pacifista, the results were very different.
an extended description!