Kas are shadow spirits born from a person's magic. They grow depending on a person's will and help channel their magic and guide them in the Shadow Realm. Egypt's royalty was unique as they didn't have kas. Expected to host the god spirits they were raised to manipulate the shadows directly.
Seth had discovered that his inheritance allowed him to sever the bond between ka and owner and use the ka for himself. In that way many criminals lost their ka, the shadow spirits becoming servants to the kingdom. Seth heard of a particularly powerful shadow spirit on the loose and terrorizing a small village. He went to investigate, and was shocked to learn that the ka belonged to a small frightened girl. He protected her, helping to calm the ka's rage at the girl's fear and pain and give her a better life. It wasn't long before he fell for her, despite their differences in station.
It wasn't meant to be. During the battle against Bakura the girl summoned her ka to protect Seth. She was mortally wounded in the process. Her ka, a great dragon, stood by until the end, accepting the girl's soul to bond with her. Seth's bond with the girl transferred to the dragon. While the shadow spirit now had faint memories of what being human was, she was still a shadow spirit, and a dragon. So the ka did what she could to help.
Instead of becoming just another shadow spirit the dragon bound herself to Seth to serve him. Their bond was powerful, enough that when Seth fought the pharaoh over his decision to seal the Shadow Realm she was the one to fight against the god spirits' host. She was angry to be sealed from her master when the Shadow Realm was closed off, but watched him and waited until when she could be with him again.