AppearanceKuro 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.
PersonalityIn general, Kuro is a genius of the highest caliber (having the second highest IQ in East Blue, behind Ben Beckman), whose plans have never failed in the past. 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 Hundred 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.
He also seems to like the glasses he wears very much, even though they tend to slip down his nose every so often. He pushes them up with his palms so he doesn't scratch his face with his Cat Claws, a symbol of remembering his murderous ways. Even despite the lens being shattered by Luffy's punch, he kept them on. The glasses are his signature belongings, and as such, he destroyed the new ones Kaya gave him (though it may be because he simply hated her). Upon Kuro's defeat, the glasses dropped onto the ground, bent, in a symbolic manner.
Kuro follows the role of the evil mastermind and continues One Piece's tradition of noticeably 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. Judging by the severe change in personality between Captain Kuro and Kurahadol, it is possible that Kuro has some degree of a split 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 psychological reasoning. On the night of the Marine attack, there was once again a crescent moon, which seems to have been the reason why Kuro was so brutal to the Marines on board.  As a consequence during the first opening of the anime a red crescent moon can be briefly seen in the scene with Kuro.
“ Pirates should be faithful pawns of their captain. They live and die by my command.
-Kuro on crew mates
Kuro had 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 than 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 three 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.
AbilitiesIn 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. While he uses super-human speed, his weakness (being unable to tell what he is hitting) puts him at a set back against other super-speed users who can tell what they are striking, though the fact he can move so fast at his level (which is weak compared to other super speed users) is still impressive. His inhuman speed is due to his powerful legs; therefore, he also possesses powerful kicks, as demonstrated when he kicked Usopp and Luffy sending them both flying for a distance.
Kuro is also knowledgeable about the existence of Devil Fruits, especially as a citizen not from the Grand Line, most of whom would not even know about Devil Fruits, let alone accept their existence as more than a myth, as he knew Luffy's rubber powers derives from it.
WeaponsCaptain Kuro uses weapons called "Cat Claws", furred gloves with a full-length katana blades at the end of each finger.
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.