He was the first member to join the crew, and is considered one of the Monster Trio (怪物三人組,Kaibutsu San-ningumi?) alongside Luffy and Sanji. His fame as a master swordsman and his great strength, along with the actions of his captain, sometimes led others to believe he was the true captain of the crew before he obtained his first bounty.
His dream is to become the greatest swordsman in the world. Zoro is also regarded as one of twelve pirates who are referred to as the "Worst Generation". He currently has a bounty of 320,000,000, which is the 4th highest in the crew, after Luffy, Jinbe, and Sanji.
Zoro is a muscular man of average height with lightly tanned skin. He always carries his three swords along with him, bundled up with a green haramaki over his right hip, allowing him to easily draw them with his left hand. Of the three swords he has on his person, he has always been seen carrying the Wado Ichimonji, while his other two swords have varied over time due to them being destroyed and subsequently replaced with progressively better ones. Although his right hand is usually seen resting over them, he can also use this hand to pull his swords out before sitting down. He has three identical gold earrings on his left earlobe. Zoro normally keeps a black bandana tied around his left bicep and ties it around his head when he is fighting seriously against an opponent.
Zoro's body is covered with scars from his many battles, most notably the massive scar stretching from his left shoulder to his right hip that he acquired during his loss against the world's greatest swordsman, Dracule Mihawk, at Baratie. His other visible scars are the scars he inflicted upon his ankles in a desperate attempt to free himself from Mr. 3's Giant Candle Service Set by trying to cut his feet off at Little Garden. After the timeskip, he gained another scar over his left eye, obtained through unknown means.
Zoro's other remarkable feature is his cropped green hair, which Sanji often teases him about by calling him "marimo" (moss-ball, moss-head in the Viz Manga and FUNimation dub) due to its resemblance to one.
Similar to most of his male crewmates, Zoro has the same outfit throughout most of the series. Before the timeskip, he would consistently wear black trousers tucked inside black boots, a plain white shirt with three undone buttons at the collar and his green haramaki. However, only Zoro's shirt varies from time to time in the arcs.
During the Arlong Park Arc, he disembarked Sanji's fishing ship neglecting to put on a shirt. After beating up most of Arlong's crew, he wore an opened, navy blue shirt decorated with light blue whirlpools that he stole from one of the fish-men. Since Zoro received his scar from Mihawk during the Baratie Arc before the Arlong Park Arc, Zoro wore bandages that were visible underneath his shirt.
During the Drum Island Arc, Zoro was bundled up for the winter island, but later stripped down after a foolish choice to swim in the frigid water on the island. He remained shirtless and shoeless in the beginning of the arc after getting lost from the Going Merry and left unable to redress himself, until he stole a parka from a man in Wapol's army, which was a hooded, forest green parka with a light green sash and white fur on the cuffs, hood, and hem, as well as a pair of boots.
During the Alabasta Arc, he wore an orange keffiyeh with a gold chain on his head, a long-sleeved white coat with black lines all over it, and the bottom of his black trousers were torn above his ankles.
During the Skypiea Arc, he wore his usual outfit in the beginning of the arc until he took it off after fighting a skyshark to save Tony Tony Chopper from being eaten. Afterwards, he wore a long, deep blue T-shirt and goggles strapped either on his head or around his neck, which were later broken and discarded.
During the Water 7 Arc, he wore his usual outfit until near the end, where he wore an unzipped, bright yellow jacket with a closed pocket flap on each side. He continued wearing this outfit throughout the Enies Lobby Arc.
During his time at Kuraigana Island, shortly after being sent there by Bartholomew Kuma, he wore an opened, dark blue shirt. Since Zoro was critically injured when he arrived at the island, Perona patched him up, so bandages were all over his body.
During and after the two-year timeskip, Zoro gains a new scar running down over his left eye, which is now implied to be critically injured as it is always closed. His frame gets larger, showing considerable more muscle mass and his hair gets shaggier while slicked back. Zoro now wears a long, open dark-green coat closed on his waist and is held by a red sash, in which his three swords are now tucked into, and his black bandanna is tied around his left forearm. His coat exposes his bare chest, revealing his green haramaki underneath the coat. He has also grown noticeably taller, as he is similar in height to Robin.
During the Dressrosa Arc, he wore a black suit and a white dress shirt with the top button undone. He also wore black sunglasses and a fake white mustache as a disguise. His sunglasses were destroyed in his clash with Admiral Fujitora. Additionally, he discarded the mustache upon his identity being exposed. After the battle against the Donquixote Pirates, Zoro took off his suit jacket, and unbuttoned his shirt leaving his chest and scar exposed.
During the Zou Arc, Zoro wore a dark-blue t-shirt with his green coat and haramaki over a pair of long, black trousers and boots.
During the Wano Country Arc, Zoro wore a white yukata with black details and a green cloud pattern on its bottom. He wore a dark green jittoku haori with a mon of a black sun over a khaki gear, and zōri with a black strap. Zoro also styled his hair in the chonmage style.
In the Movie 2, after the Trump Pirates stole the Going Merry with his clothes inside, he had to wear a black kimono from a wedding store. Then he changes his clothes for an open black jacket, a white scarf, and black pants like the ones he normally wears.
In the Movie 3, he wears an open blue jacket with short sleeves, with a white line on the sleeves, short black pants, and still wearing his green haramaki. Also momentarily, he wears an extravagant penguin costume designed by Usopp.
In the Movie 6, he wears black pants, and a brown shirt, which he later exchanges for an open white jacket.
The reason why it is depicted with some green has nothing to do with a color change, if you will notice most of it is still black. The green in the anime is an effect used in cartoons and comics to show shading/highlighting, as the color black is incapable of doing so itself. Light shades of purple, gray or blue are sometimes used instead. White, red and yellow are often too bright to pull off the same effect. Oda does the same effect in color walks.