Ryuk is the main Shinigami of the Death Note series. Bored with the activities (or lack thereof) of the Shinigami realm, Ryuk obtains a second Death Note and drops it in the human realm for someone to find, hoping to amuse himself. He deliberately writes the instructions on the front page (in English, which he assumed to be the most popular language in the human realm) so people would understand its purpose. The Death Note is discovered by Light Yagami, and Ryuk follows him around for much of the series to see how Light uses it. Ryuk has a fairly humanoid appearance. His skin is gray, or a purplish color, his limbs are abnormally long, and he has bulging yellow eyes with black irises (red in the anime).
Ryuk is characterized by his constant amusement by, and apathy in regards to, Light's problems. He enjoys seeing Light overcome the various challenges put to him, and often waits until the worst possible moment to inform him of a certain aspect of the Death Note just to get a laugh. He is occasionally helpful if it serves his own interests, such as obtaining apples or furthering his own amusement, but for the most part will jokingly ask Light what his next move will be or have Light explain to him the point of a certain action. The sfist describes Ryuk as a "crazy-looking, supernatural creature with these wicked eyes" who becomes "(surprisingly)" a "moral compass" for Light in the film version of Death Note.
Ryuk has a great fondness for apples, comparing them to cigarettes and alcohol for Shinigami (Shinigami apples are withered and taste like sand, as he shows Misa at one point), and will go through a type of withdrawal if he goes for too long without eating them. His withdrawal symptoms involve twisting himself up like a pretzel and doing handstands. He also states that he is shy around girls.
As Ryuk explains when he first meets Light, he is bound to take Light's life when his time comes. In the manga, Ryuk does this after Light is shot several times by Matsuda. He desperately begs Ryuk to write the names of the investigation team and the SPK members in the Death Note, but Ryuk, reminding Light that he is not on anyone's side, just writes Light's name instead. He was expecting Light might have thought of some way out of his situation, but seeing as he was desperate enough to go to Ryuk for help, he decided it was all over for him. Ryuk returns to the Shinigami realm.