Elements of "D.Gray-man" first appeared in Hoshino's one-shot title "Zone", in which the Akuma, the exorcists, and the Millennium Earl plan to end the world. Although Allen Walker is male, his character is based on "Zone" female protagonist. Lavi is based on the protagonist of Hoshino's planned series, "Book-man", that she originally wanted to write. Other characters, such as the Millennium Earl, Lenalee Lee, and Komui Lee, are based on real people whom Hoshino has not specifically identified; some are well-known scientists, and Komui is based on Hoshino's boss. The character of Yu Kanda, based on a samurai, was created to vary "D.Gray-man"s Western setting. Hoshino found the design of some of the characters difficult early in the series. In 2011, the author visited New York City for research, and believed that the city had greatly influenced her work. Hoshino visited cemeteries, and was deeply impressed by her guides' comments at Ground Zero of the World Trade Center (left after the September 11 attacks). She said that she would like to spend more time in New York City to gather data for the series.