A majority of Information on this page is speculation and has not been confirmed by canon sources
Throughout the games that make up Umineko no Naku Koro ni, the ultimate perpetrator of the deaths of the Ushiromiya family employs a number of accomplices among the family and its retainers by buying them out with the gold and threatning them with the bomb. These accomplices are used to provide alibis for potential suspects, transmit messages to the survivors via envelopes, and falsify details of the crime scenes. In general, they attempt to impress upon Battler the "fantasy" line of events, in which the murders are committed by the supernatural agents of the Golden Witch Beatrice.
The extent to which the accomplices believe the murders are real varies. Throughout the series, some are convinced the murders are faked, while others are almost certainly aware that they are real.
These three swore to accept Beatrice as the new head of the Ushiromiya family following Kinzo's death, and assist the witch in all fragments. Genji seems to be the most willing to aid Beatrice, as Nanjo shows somewhat conditional loyalties (particularly in Turn) while Kumasawa doesn't seem to believe the deaths are actually real.
Although her influence as an accomplice is rather limited, she assists Beatrice by delivering the first letter to the adults in all games, and is generally willing to do whatever Beatrice asks her to do.
Our Confession details Beatrice's methods of constructing the murdery mystery, and how she recruits accomplices among the family. In the TIP, the servants escort Krauss and Natsuhi to the underground tunnels, where Beatrice reveals the existence of the gold, and the bomb that will annihilate the island if anything goes awry with her plan. The only escape is via the tunnels to Kuwadorian, through a locked grate that she holds the key to.
After the first twilight, Eva and Hideyoshi both act to prevent other members of the family from closely inspecting the shed where the bodies are found. Hideyoshi in particular falsifies the appearance of Shannon's body in the shed among the other five, with some prompting from Kanon. In conversation with Battler, Eva refers to the extensive facial damage of the corpses as "horrible make-up", indicating that she believes the deaths are faked; Eva and Hideyoshi's casual dialogue about vacation plans when separated from the rest of the family also indicates both believe no murder occured and that their money troubles are over. Her attempts to use the murders to blame or otherwise one-up Natsuhi seem to make her a liability, and Eva and Hideyoshi are both slain in the second twilight.
Rosa's involvement as an accomplice begins with the first twilight; possibly betrayed by a fantasy scene which shows Rosa among the other adults accepting Beatrice as Kinzo's heir. When Battler arrives at the chapel, she tells him that she used the key given to Maria by Beatrice to unlock the chapel door, inside which the bodies are found. Although the whereabouts of the key are consistently accounted for by Red Truth, nothing ever states the chapel door was ever locked. Unlike Eva in Legend, Rosa is aware the deaths are in fact real. As the surviving adult and gun-holder for this episode, she separates the survivors several times into "wolves" and "sheep," isolating key individuals for the later twilights. After the second twilight, she provides an alibi for Shannon, indicating the two of them were speaking to Kinzo (who is dead before the beginning of all games) when the murders were occuring. She ultimately dies attempting to escape the island with Maria; with Yasu dead and the locked grate inaccessible, the two unsuccessfully attempt to reach the ocean.
Krauss Ushiromiya and Natsuhi Ushiromiya
After the first twilight, during which they had held a "strategy meeting" with the other adults regarding Beatrice's letter (in the other fragments, the adults break off the meeting and go to sleep), Krauss and Natsuhi lead the rest of the family around the mansion to discover the bodies of the servants, moving in an order that gives the perpetrator time to set up a circle of locked rooms. The only discovery shown in detail is that for Shannon, where Krauss prevents the others from getting close to the body until Nanjo can "examine" her. Natsuhi maintains an unusually calm response to six murders occuring in the house, as opposed to her demeanor in Episode 1 and later in the series. She is also very fast to point out that the keys to five rooms and the chapel are missing, and that the burned body in the boiler room possesses six toes on each foot. Due to Eva solving the Witch's Epitaph, it is implied Beatrice's plan goes off the rails at this point, and the two are murdered much later in the story.
These two communicate with Battler by way of telephone, describing the manner of their imminent death and urging Battler to believe in the witch.
Due to the nature of this episode, it is possible other members of the family (and Gohda, who answers the telephone for "Krauss" when he contacts Jessica) were also cooperating with Beatrice, however none of them communicate with Battler afterwards.