During it's run, there were many fan theories attempting to find the solution to the mysteries of Umineko no Naku Koro ni. Listed below are some of the most popular of those theories.
Shkanon
The Shkanon theory postulates that Shannon and Kanon are either the same person or are simply working together to commit the murders on Rokkenjima. This was proven correct with the existence of Sayo Yasuda.
Rosatrice
The Rosatrice theory is an alternative explanation of the murders on Rokkenjima in which Rosa Ushiromiya is the mastermind with George Ushiromiya and Doctor Nanjo as her primary accomplices. It gained notoriety among the fanbase due to a nine-hour long, five-part series of YouTube videos by user "KnowNoMore" which went in to in-depth explanations on how the theory worked.
Clumsy Gohda
Until we reach the Golden LandA rather humorous theory postulating that Toshiro Gohda was involved in every murder on Rokkenjima, committing them accidentally through a series of mishaps. This theory also makes use of the Small Bombs theory below , claiming Gohda accidentally used a box of "Tiny Food Bombs" instead of sugar while preparing snacks for the Halloween party. When Battler Ushiromiya learns the "truth" of the murders, he finds it too ridiculous to believe and instead decides that it is more likely that a witch did it.
Lambdadelta is Bernkastel's companion voyager witch and thus is the subject of speculation regarding a possible human identity. The most common version of this theory is that Lambdadelta is Miyo Takano, main antagonist of Higurashi no Naku Koro ni and Rika's main rival. This is supported by the similar hair between the two characters. Another common speculation is that Lambdadelta is Satoko Houjou, Rika's best friend. This is supported by similar hair colors between the two as well as the similarity of Rika and Satoko's relationship to Bernkastel and Lambdadelta's relationship. Ryukishi07 has dismissed both theories as red herrings.
In-game theories
Small Bombs
The Small Bombs theory was postulated by Battler Ushiromiya in Turn of the Golden Witch. Battler suggests that the first twilight, wherein the victims were found with their intestines ripped out sitting around a Halloween party table, was a result of the culprit feeding the victims "small bombs" in their meals. The absurdity of the theory has led it to be a subject of ridicule with fans.
In Banquet of the Golden Witch, Battler posits the idea that Eva Ushiromiya was responsible for each murder. For the second twilight, she snuck out of her room to kill Rosa and Maria Ushiromiya. Her husband, Hideyoshi Ushiromiya, remained in their guest room to create an alibi. For the fourth, fifth, and sixth twilights, Battler posits that Kyrie Ushiromiya led a group to the mansion under the guise of retrieving food but was actually suspicious of Hideyoshi as she found a cigarette butt in the ashtray of his and Eva's room. Since Eva hates smoking, it is unlikely that Hideyoshi would've smoked in their room if Eva had been present.
