Kuro is a man of relatively slim build with glasses and slicked green-and-black hair. During the present storyline, he wears a standard butler's uniform with striped shoes (which he apparently kept from his life as a pirate). During the time when he was a pirate, however, he wore the standard "captain's jacket" with the Black Cat pirates' symbol on the shoulders. Oda Sensei says that the golden design on his jacket is actually poop.
In general, Kuro is a genius of the highest calibur (having the second highest IQ in East Blue, behind Ben Beckman), whose plans have never failed in the past (making him similar to Crocodile). He is a master strategist and an accomplished fighter, but he was severely hampered by the fact that he simply didn't have the fortitude to deal with a pirate's life on the run from the Marines.
Kuro seems to be very proud of his plans, which never failed before (hence his nickname, Kuro of a Thousand Plans). When he relived his moments of piracy, he claims his plans were too brilliant to be wasted on his crew. He also said that he would kill his crew if they hinder his plans in anyway. During the final moments before Luffy defeated him, Kuro lost his usual calmness and screamed aloud that his plans would never fail.
Kuro follows the role of the evil mastermind and continues One Piece's tradition of noticably cruel villains. He held himself in very high regard, having a standard for people no one seemed able to live up to but himself. He felt nothing for the lives of others, willingly sacrificing anyone he needed to achieve his goals.
During his three years as Kaya's butler, he acted as a kind-hearted (though a little snobbish), trustworthy person who carries a hatred for pirates, and as such, looks down on Usopp. He later admits how much he despises this pseudo-personality.
Though able to keep himself in check most of the time, he states that seeing a crescent moon drives him wild and even he finds it difficult to resist the temptations of violence brought on by the stage of the moon.  The reason is never given for this, though it may be due to a psychological effect. On the night of the Marine attack, there was once again a cresent moon, which seems to have been the reason why Kuro was so brutal to the Marines on board it.  As a consequence during the first opening of the anime a red crescent moon can be briefly seen in the scene with Kuro.
He has come to the conclusion his crew were nothing but puppets; that pirates were barbarians that stole from people out of greed and nothing more, and that they were buffoons who were nothing without his guidance and direction.  To him they were seen as nothing more then this. He would willingly sacrifice them all and by the time his plan was ready he had already long decided they should all die.  This enrages Luffy and makes him even more determined to stop Kuro.
His life as a pirate led him to become irritated at the idea that he was forever chased by the Marines, as well as those seeking his bounty. He then began to yearn for a peaceful life, without such bothers.
As Kurahadol he was forced into acting kind natured, to make himself out as Miss Kaya's trustworthy family butler and caretaker. He later admits how much he hated acting out this role for 3 years, and in the process betrays Kaya's feelings. At one point, he relives seemingly happy memories with her, breaking her heart as he states that he "endured it", and voiced his utter contempt for her. He mocks Usopp when he calls himself a pirate, calling his act a "Childish game", and sees him as equal to an insect.  Usopp dislikes him for this, and comes to hate him after realizing what he is planning.
In addition to his high intelligence, Captain Kuro has super-human speed with which to deliver his attacks. His skill is such that at full speed he is only able to be seen as a vague blur or by the damage his attacks cause. He is able to run 100 meters in between 4 and 5 seconds..
Captain Kuro often pushes his glasses up on his nose with the palm of his hand, a technique he has used since his days as a pirate - straightening them with the fingers would be impossible when the Cat Claws are worn. According to Jango, this proves that even after three years of peaceful life Captain Kuro has not yet forgotten how to kill.
The Black Captain
The dreaded former Captain of the Black Cat Pirates, he is also known as Kuro of the Thousand Plans for his elaborate plans that virtually never fail. However, this proved to have a huge setback; as time passed, he grew tired of setting up schemes for his crewmates and of constantly being hunted down by the Marines.  That was when he decided to have an impostor take his place as Captain Kuro and be executed in his stead by Morgan so that the hunt for him would be ceased. 
Soon after, Captain Kuro set up an elaborate scheme in a small village he came to live in, in order to live free from pursuit with great wealth. He took the name Kurahadol and, for two years, lived as the butler of the wealthy parents of Kaya, who he is believed by many to have killed (this is not known to be true yet, as he denied it when Django accused him of it, saying that their deaths weren't part of his original plan). He then became the sickly Kaya's caretaker.
When three years passed, he contacted his old first mate, the hypnotist Django, and arranged for the Black Cat pirates to lay siege to the village, force Kaya to write and sign a will leaving all her wealth to Kuro, and then kill Kaya. He would then kill the pirates who attacked the town to make it seem as though they had killed her in their attack. His plan goes awry when he is overheard by Monkey D. Luffy and Usopp when he speaks to Django about his plans. 
Scratch Marks on the Coastline
Luffy, Usopp, Roronoa Zoro, and Nami fight togther against the Black Cat Pirates. Usopp saves Kaya from Django's pursuit in the nick of time, Zoro defeats the Nyaban Brothers (Meowban Brothers) (Sham and Buchi), and Luffy defeats Captain Kuro, using Gomu Gomu no Kane/Gum Gum Bell (A stretched headbutt) after immobilizing him by wrapping his rubber limbs around Kuro's.  
Kuro was last seen (in the anime only) in a cameo appearance on board of his ship, as the captain again, when his crew informed him of Luffy's new bounty.
Translation and Dub Issues
His name translates to "Captain Black".
Due to his odd alias, Kuro's "butler name" has been translated in a wide variety of ways. The English adaptations use the name Klahadore, possibly a "claw" pun. The pronunciation of "Klahadore" could be compared to "claw-a-door" in English. Most fans of the original version, however, use the term Kurahadol; as it resembles his real name, and due to its pronunciation in the Japanese version of the anime (emphasis on the "ku-ra", a light roll of the "ru" at the end, signifying a Japanese attempt at an L sound).
- When he was in his butler attire, there were strange symbols on his jacket. Oda jokingly explained that these symbols are poop, to signify that he is lying. This directly resembles the American slang term "Bull Shit", meaning lie. Although the answer was obviously a joke (as Oda is known to give joke answers to questions he can't seriously answer), this could be a play on the slang term "full of crap".
- In an SBS in the manga, Oda stated that the most intelligent person in all of East Blue is Ben Beckman, followed by Captain Kuro in second and Nami in third.
- In a recent Japanese Fan Poll, Kuro is currently ranked the 24th most popular character in One Piece.