Shidoh is a Shinigami whose Death Note is stolen by Ryuk. In fact, Ryuk takes his Note and gives it to Light, which becomes Higuchi's note and is finally passed on to Mello's gang (more specifically, Mello's subordinate, Kal Snyder (also known as Jack Neylon)). Shidoh has a great liking for chocolate. Since Shidoh has neglected to kill people and extend his own lifetime, he probably did not notice that his Note is stolen. As the end of his lifetime approaches, Shidoh finds Ryuk and learns that Mello is in possession of his Note. Later, he finds Snyder in the human world. To regain his Note, Shidoh agrees to help Mello's gang against the SPK and Light. After Light regains the Death Note from Mello, he returns the note to Shidoh for him to stop interfering and return to the Shinigami world. Although these events occurred over the course of about one volume in the manga, Shidoh was only in two episodes in the anime version (episode 28 where he was merely introduced in the last few minutes of the episode, and episode 29, which relays most events of Shidoh's regaining of his Death Note).
Shidoh's name is written in English as "Shidoh" in Death Note: How to Read 13, but written "Sidoh" in both the tankōbon character introductions page and within the translated English manga.