Seiji's father married into the Date family (meaning he took the last name of the wife's family instead of the wife taking his last name) and practices kendo as well as being a police officer. He IS Japanese because it is very unlikely for the Date family to let foreigners marry into the family. He has a weaker personality than his wife.
His mother is probably the eldest Date daughter and is second in the family dojo. Only her father outranks her. She is a teacher in the dojo and has a stronger personality then her husband. (Women are more powerful and are of higher rank in Seiji's family.)
His grandfather is a very harsh, strict old man in charge of the dojo and Seiji's upbringing. (Imagine living and putting up with a cranky old man every day! Poor Seiji-kun!) His grandfather taught him the way of the Bushi and how to keep his temper under control seeing that Seiji was quite rebellious when he was younger.
Yayoi is at least four years older than Seiji. She practices kendo, speaks formal Japanese, and is a medical student with a wicked sense of humor (Oh, I'd like to give her a piece of my mind for teasing Seiji-kun!). Yayoi often used Seiji as a dress-up doll, and even has a picture of him with ribbons in his hair. She is the prime reason Seiji dislikes talking to girls. Because women in the Date family are more powerful and in charge than the men, and because Yayoi was an elder, there was really nothing Seiji could do about it. Yayoi also has black hair.
Satsuki is at least two years younger than Seiji and does not practice kendo. She doesn't speak a formal language of Japanese and calls Seiji Ni-chan, meaning 'brother'. Satsuki also has black hair. She is possibly the reason for Seiji being protective of women and children.
Seiji was quite rebellious when he was younger. It wasn't that he didn't like kendo, he didn't like all the etiquette. To teach him restraint, his grandfather made him study bonzai and the bamboo flute in order to teach Seiji how to calm himself down.
When Seiji was a child, he was very sick. I think he was even hospitalized once. I'm not sure about that though. Anway, he was sooo sick, his parents dressed him up as a girl following a theory that girls live longer than boys. It was a long time ago and Seiji doesn't remember this. I think eventually, in the Korin Den CD, he is finally told about this after he is run over by a motorcycle protecting a young girl's life and hospitalized. I believe his doctor, after Seiji wakes up, tells him about his days being dressed up as a girl. Apparently, based on the statement by the doctor, it wasn't unusual for parents to dress their sick children up because it follows some sort of tradition. Of course, all this I am not COMPLETELY sure of, so maybe you want to read the Korin Den CD first. ^^
Seiji scared other children when he was younger. Seeing both of Seiji's pale eyes made children cry. His older sister, Yayoi, told him he should cover one eye with his hair so he wouldn't frighten them. And tada! The Date Seiji look was born.
During Seiji's rebellious childhood, he once took the family's sacred sword and played with it in the courtyard. While playing, he broke one of his grandfather's bonzais. His grandfather never noticed because a tulip was placed there. Seiji dreaded the punishment that would come if his grandfather ever found out. (Poor Seiji. His grandfather must have been really rough on him!!!)
Seiji's family, Date, is descended from Date Masamune. He is a famous general who built a castle in Sendai, the capital of Miyagi. He only had one eye and was often called the One-Eyed Dragon. Seiji may live on the outskirts of Sendai, in the family dojo, which he will one day inherit. He is known as the young master.