Asato Tsuzuki is the main protagonist of the story. He was born in 1900, and was 26 years old when he died and became a Shinigami. Though he dies at 26, he looks a little younger, probably due to dying in a hopital after 8 years. He is 97 at the beginning of the first book. He is the oldest employee of the Shokan/Summons division, besides Chief Konoe, and the lowest-paid, due to his perceived incompetence.

Tsuzuki enjoys sake, apple pie, cinnabon, teasing Hisoka, and sleeping. He is notorious among his fellow Shinigami for his slacker qualities and voracious appetite. His favorite color is light green, and he has a flower garden (in which he is known to have tulips and hydrangeas).

Starting in the second volume, in Last Waltz, we learn more about his relationship with his sister. In Tankoubon 5, Storybook, he gives his nameless female role, who had been known only as 'girl,' his sister's name of Luka. She had brought him to be fond of dancing and gardening, at which he's quite adept, and cooking, which is about as edible as poison. These culinary attempts always lead to some sort of bad reaction, as shown in Volume 4, Hokkaidō. (The Queen had gone missing, and upon finding her later it is learned that she had eaten a muffin which Tsuzuki made, that Yuma and Saya accidentally left behind. It was the cause for her disappearance, while she really couldn't recall how she ended up out of her 'kingdom.')

Throughout the series, Tsuzuki develops an immediate closeness and affection to his current partner, Hisoka. He has a good friendship with Watari, and a sometimes strained relationship with Tatsumi, who had once been one of his partners. Tsuzuki gets along with most of the employees of Meifu, with the notable exceptions of Hakushaku, who constantly hits on him, and Terazuma, with whom he has a fierce rivalry. Tsuzuki's relationship with Muraki is very tumultuous; although Tsuzuki hates him for his cruelty toward Hisoka, himself (and, actually, pretty much everyone he meets), Tsuzuki's desire to sacrifice himself rather than hurt anyone else prevents him from killing Muraki in cold blood.

Though he is easily one of the most cheerful members of the cast, he hides a dark secret from his past. Both manga and anime reference terrible deeds he committed in life. It is suggested that Tsuzuki murdered many people, in some way or another, whether intentionally or as a by-product of something else; this is brought to Tsuzuki's attention during his demonic possession by Saagantanasu, a powerful demon who appears in the Devil's Trill Arc. Doctor Muraki reveals to him from his grandfather's research that Tsuzuki was a patient of the elder Muraki for eight years and that Tsuzuki is, in fact, not quite human. During that time, he stayed alive without food, water, or sleep, and was incapable of dying from wounds, as shown by the many times he tried to commit suicide but failed, but for one last time. Muraki suggested that Tsuzuki may possess demon blood [shown by the fact that he has purple eyes], and as Tsuzuki found this incredibly hard to deal with.

Tsuzuki wields the power of 12 Shikigami as well as ofuda magic. He also has an incredibly high stamina, able to take massive damage to his body without being killed and healing almost immediately. Though later, this was shown to be a trait for all Shinigami, he was the first one shown to have this ability, which appears to be tied to his pre-death abilities.