Watari's name was originally going to be "Shadow" in reference to being L's shadow, but the editor disapproved, which led to the name "Watari", which references his role as a handler. The design team decided on Watari's design during the second chapter. Obata did not put much thought into Watari's disguise because his face did not appear, but afterwards, the rough drafts had Watari as an elderly man with a single strand of hair on his head. Obata decided to keep him as an old man because he believed older characters were more fun to draw than younger ones because of the "weird angles" that could be created with their wrinkles, while younger characters may be designed as "attractive, normal, or ugly." Obata likes leather coats, so he had Watari wear one. Watari is Obata's third favorite human character because he likes older characters and that because Watari "seemed to be hiding a lot of ability" and "had depth." His birthday is May 1, 1933.