Byakuyamaru Natsugawara (夏瓦白夜丸, Natsugawara Byakuyamaru) is a childhood friend of Shingen Kishitani and the current owner of the Natsugawara Group, a toy company, as well as the Natsugawara Summer Tile Baseball Stadium.
Generally outspoken and good-natured, his sense of ethics is one of the few questionable things about him.
Byakuyamaru is described as being young in appearance but dressed in the stereotypical manner of a wealthy person.
He first encounters Celty following his son, Yukihiko's, theft of a certain SD card containing erotic images. Celty attempts to call him out for committing multiple felonies, but he firmly asserts that there are certain things he would never do due to his pride in his business, claiming that his business was made to bring happiness to children and make their dreams come true.
Although he appears to have enough pride as to not commit certain questionable acts, his morals are brought back into question once Celty discovers that his company has been engaging in human experimentation. The SD card Yukihiko had previously stolen was just a disguise for the actual contents.
Byakuyamaru does not attempt to lie to Celty about his company's role in the experiments, claiming that he was only trying to make any young child's dream of obtaining superpowers come true. Jami Natsugawara, one of his adopted children who comes through the experimentation unharmed, assures Celty that no harm is befalling the children and that they are not making weapons.