About Kurogane
Kurogane is actually one of the few original characters (as in, not a crossover character from another work) in TRC who has an important role in the story. Also, while we can't call him the protagonist or a mere sidekick, he's definitely one of the five main characters (yes, I count Mokona Soel Modoki as a character XD) of the series.
Kurogane used to be a 'ninja' who worked as a bodyguard in the Shirasagi castle of the Japan Empire -not our Japan. He was under the orders of the miko princess Tsukiyomi (whose real name is Tomoyo), younger sister of the empress. One night, after killing all the men who invaded the castle, Tomoyo summoned him and showed him her disappointment about his violent ways. Kurogane is the strongest warrior, perhaps the best swordsman in the empire. No one there can win him over a fight. Therefore, the princess decides as a punishment to send him far away, in a journey, so he learns the real meaning of strength. Also, she casts a spell on him: his force will decrease in inverse proportion to the number of persons he kills from now on.
With her magical powers, Tsukiyomi sends a very angry Kurogane to another dimension. He lands (more like erupts from the ground) on a garden, rain falling over him. He finds himself surrounded by strangers: two teenagers: a boy holding to a sleeping girl, a fair-haired man with a heavy coat and a golden staff, and a mysterious woman in a black and white dress.
Soon he learns a couple of things: the woman is a witch, the place is a shop to grant wishes, the world is also called Japan but it's different, and the only one who can help him to return home is said witch. However, the boy (Syaoran) needs to find the girl (Sakura)'s memories through the dimensions and the other man (Fay) wants to get away from his own world. Four persons, the same expensive wish: to travel.
Yuuko, the witch, proposes a deal. If everyone can pay her their most important possession, they can travel and handle the price all together. Kurogane is forced to hand out his sword, Ginryuu (Silver Dragon), to which he holds an emotional attachment. After some bullying from Yuuko, he accepts, and warns her he'll be back to retrieve it.
Yuuko gave them a small, white creature called Mokona Modoki. The little animal can transport them through dimensions, translate all languages (think of the Babelfish in the Douglas Adams' books), track Sakura's memories, and some other 105 special abilities Kurogane doesn't really give a damn about. And so, the four adventurers start their journey.