Well, we can't all have talking swords. Especially not ones that run our bodies and have a lot to say about a great many things. But Kou does. Where did he get it? Long story. Byakkoshinken was being carried about by Beijing Opera actress Shoukyou, who apparently was his custodian. She and Guhon (Rin's great-uncle) stabbed the blade into a stone at a temple, in order that it be found by the sixth Ranger, the secret Kiba Ranger. And he called the Kiba Ranger with his telepathic voice. He is a star-sword. He can fly through the air. He can control Kou's movements and goes to a great deal of trouble to disguise the boy as a short adult when in armor. He teaches Kou how to fight and basically is the boy's companion and confidant. One thing not surprising is that he knows Kaku. However, Byakkoshinken also knows Kou's mother. He was an integral part of her spell to rid Kou of the Gohma influence, to allow him to live a life free. I was never clear on the links between who and when. Did Kou's mother make Byakkonshinken, in order to give him to her child? And then when she bore twins and changed her entire life... found the sword drawn to the one child but not to the other? Who knows. But the sword would not have gone to anyone else but Kou. As we've noted amongst the Gohma, the power goes to the blood-claim.