|First Appearance||Alliance of the Golden Witch|
|Portrayed By||Takayuki Kondō|
Ootsuki (大月 Ōtsuki?) is a college professor and foremost member of the Rokkenjima Witch Hunt, an international organization of enthusiasts analyzing the occult aspects of the Rokkenjima massacre. Ange Ushiromiya interviews him in Alliance of the Golden Witch in an attempt to learn the truth of Rokkenjima's destruction.
Officially, Ootsuki is a university professor known as an authority on Western folklore. In truth, Ootsuki is a noted occultist whose life's work revolved around Western magic, alchemy, and occultism. In April 1987, Ootsuki was called by a friend to analyze a collection of occult books being sold at auction. These books were former possessions of Kinzo Ushiromiya known as the "Ushiromiya Library".
Ootsuki appears as a middle-aged man with curled brown hair. He wears a navy blue suit with an indigo tie.
Alliance of the Golden Witch
In Alliance of the Golden Witch, Professor Ootsuki is visited by Ange Ushiromiya masquerading as a member of the Sumadera family. Ange inquires about the message bottles and uses Ootsuki to confirm that the handwriting in the message bottles matches that of a message Beatrice wrote in the diary of Maria Ushiromiya. After Ange leaves, it is implied that Ootsuki is tortured by Kasumi Sumadera for allowing himself be deceived by Ange.
- Ootsuki is meant to represent the reader of Umineko no Naku Koro ni.