Watari is L's assistant, providing logistics to the investigation team. Before the team is formed, he is the only person who has seen L, and the only one capable of contacting L directly. Like L, he represents himself with an Old English "W" on computer screens. He is also a fatherly figure to L. On the surface he is Quillsh Wammy (キルシュ・ワイミー Kirushu Waimī?), a famous inventor and founder of Wammy's House, an orphanage for gifted children in Winchester, England. Watari is well trained in espionage and marksmanship. Rem kills Watari, along with L, in order to protect Misa. At the end of the manga, Roger Ruvie, the manager of Wammy House, becomes the new Watari for the third L, Near. He is voiced by Kiyoshi Kobayashi in the Japanese anime and French Tickner in the English dub (except for the film, in which Ron Halder takes over).

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 strip of hair. Obata decided to keep him as an old man because 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. Watari was always counted as a father and is often seen giving L snacks.