Kurogane comes from Japan and joins the group in the first episode, after being sent to the Time-Witch by Princess Tomoyo, who also placed a curse on Kurogane to stop him from unnecessarily killing people (if he kills someone, he becomes weaker). She sent him to the Time-Witch telling him that he would "journey to become stronger" but she's hoping he will find the true meaning of strength.
Kurogane is very serious, and therefore is easily irritated. Fai knows this and enjoys teasing him along with Mokona. Fai's main method of teasing is using cutesy nicknames, such as "Kuro-pu", "Kuro-rin" and "Kuro-woof-woof".
However easily irritated he may be, he is extremely perceptive and can quickly decipher personalities, weakness' and other attributes of the group and his enemies. He is the most powerful in the group and Syaoran asks him to teach him how to fight.
Even though he comes across as ruthless and aggressive, he is very loyal and will protect those he feels are his companions. At first, he claims he'll have nothing to do with Syaoran and his quest, but soon gains respect for him and helps him and Fai to collect Sakura's feathers.