Sidoh (シドウ, Shidō) is the Shinigami whose Death Note is stolen by Ryuk. By the time he realizes this, however, the note has already changed hands several times, ending up in the hands of the Mafia (more specifically, Jack Neylon).
Sidoh wears heavy clothing that obscures most of his features, and his head is wrapped in bandages. His mouth is a canary's beak. Obata states that Sidoh's "arms" can be described as insect-like in the way they fold into his body. Though he has "hands," there are only three fingers on each appendage.
In Death Note: Successor to L, parts of him are brown and red while his bandages are a much brighter white.
Sidoh has a great liking for chocolate, just like Ryuk does for apples, a taste he shares with Mello. He can be persistent when chasing something he wants, like his notebook. Death Note 13: How to Read describes him as "cowardly," being intimidated by Mello despite the fact that Mello is a human.
With a short lifespan remaining, Sidoh hopes to find his Death Note quickly. After repeatedly hounding Ryuk for the return of his notebook (which also involved him passing out flyers to other Shinigami in the vain hope that they might recover it for him), he tracks his notebook to the Mafia. In order to regain his Death Note, Sidoh agrees to help Mello's gang against Near's SPK and the Japanese Task Force. After Light Yagami regains the Death Note from Mello, he reluctantly returns it to Sidoh to keep him from interfering and as a way of trying to retain the trust of the Task Force.
In other media
Although he does not appear in the film Death Note: Light Up the NEW World, it is revealed in the novelization that he is one of the six Shinigami who newly bring Death Notes to the human world. He possesses Alexey lvanov and is the first Shinigami introduced in the novel. He instructs Ivanov to do Kira's work, and Ivanov uses websites to find his victims and conduct mercy killings.
After Yuki Shien kills Ivanov and takes his Death Note, Sidoh possesses him next. Sidoh's fate is unknown. Four of the six Death Notes are destroyed at the end, but it's unknown which two remain, so Sidoh might or might not still be in the human world attached to a Death Note.
Sidoh has a slightly expanded role in the 2018 audio drama, assisted in by L's spirit (now a Shinigami-like entity) in trying to recover his notebook. While searching for it, he mentions that Shinigami have become especially lazy since "the war" and that Tokyo was much different the last time he visited. They find the Mafia shortly before being raided by the Task Force, and Sidoh's notebook is returned by a dying Soichiro Yagami.
"I don't know, L. Maybe your teeth are going to get longer and longer. Or! Horns may come from your skull. Or you’ll just keep growing taller. Death is only the beginning."
Tsugumi Ohba said that he used Sidoh as another Shinigami appearing in the human world because he liked Sidoh's appearance; Ohba described Daril Ghiroza as a "candidate" and that he wanted a "pretty pathetic" character. Sidoh was chosen over Ghiroza because her high rank made her ill-suited for being pushed around by Mello.
Takeshi Obata described Sidoh as "very funny," citing the time when Sidoh becomes the first Shinigami to distribute flyers. He also claimed to have wished that Sidoh had appeared more often in the story.
Obata said that when he discovered that another Shinigami would appear on Earth, he filed through designs and nominated Sidoh and Daril Ghiroza. Obata said that he believed that Ghiroza would be chosen and prepared for that decision; instead, Ohba selected Sidoh. Obata said that he based Sidoh's design on the appearance of a bird with his mouth being based on a beak of a canary and with foldable, "insect-like" arms.
- Summary and partial translation of Episode 10: Karma Police https://mikami.tumblr.com/post/186497018591/death-note-audio-drama-10