Masashi Nakajima is the father of Youko Nakajima and husband to Ritsuko Nakajima. It is implied that he is a salaryman.

Youko's father appears only in the Sea of Shadow arc and is unnamed in the novel. Little is known about him, and he is depicted as somewhat overbearingly strict and detached, expecting his wife and daughter to be supportive and submissive to his expectations. It is mentioned that Youko's high school was chosen by her father, who wanted her to attend a school close to her house and had neither a particularly bad or spectacular reputation, in spite of the recommendations of Youko's junior high school teachers for her to attend a more prestigious high school because of her high grades. Despite her personal preference for Youko to attend another school, Youko's mother agreed with her husband's decision. In another incident in Youko's childhood, she asked her mother for a pair of jeans to wear to camp so she could run in races against boys. Her father opposed the idea, saying that girls should not dress or talk like boys because it was shameful, and demanded that the jeans be returned. Youko's mother reluctantly complied, despite Youko's attempt to protest.

After Youko is suddenly spirited away to the Twelve Kingdoms, her fate is unknown to the people back where she was born. Youko learns about how her parents have reacted to her disappearance through the visions brought on by the Suiguutou. While her mother is at a loss and hopes for her daughter's return, Youko's father accepts the idea that Youko has run away from from and has been concealing a delinquent lifestyle from her parents. When Youko's parents begin to argue, Ritsuko accuses her husband of not knowing their daughter at all and going to work without any concern for her disappearance, calling him selfish. In response, Youko's father suggests that if Youko had not run away, it is probably she was caught up in something and likely dead.

(Source: 12kingdoms.wikia)