During the first game, Beatrice is just as a keyword that everyone use in order to explain what happens with the deaths of their relatives. She acts in the shadow and she's apparently not active part of the murders.
A bit about her is told by Kinzo at the begin of the game: he says that he would do anything in order to see her sweet smile again, but she will never come again until the day... in which he will maybe meet her again after death (LOL). A giant portrait of her is in the reception hall of the house. The first impression it made to Battler, was that it was about a portrait of his grandmother when she was a girl, but, instead, he understood that because of the western features she had, it was impossible. Another portrait, but of normal shapes, is found in Kinzo's study. Many things are said about her, that she was Kinzo's concubine and that she was the person who gave him the gold of the legend. But then, it is said that she helped him a lot with alchemy and that she was, maybe, her assistant.
Rokkenjima's island servants and the people who live in that home, as Jessica and her family, know a legend that was said when the guys were children: it was said that a witch lived in the middle of the forest, and because of this, it was very dengerous to approach there. Obviously, all of them thought it was just a way to let the children away from the forest, that was a place so dangerous that, if perhaps a child should fall somewhere, no one could hear his screams. Anyway, the servants were not always sure about this, because Shannon herself, and Kanon too, said they already saw Beatrice in the form of shining butterflies in the house, since she's not a human and has not an own body. The only person who firmly believes in her is Maria, who always try to convince all that Beatrice "exists", but 'till the end, nobody will believe to her.
She will make her appearance at the end of the game. When she will praise the wise at the tenth twilight, that in this case will be Natsuhi, she will take the last four people who remained (Battler, Jessica, Maria and George) in an ideal world (Purgatory LOL), where she wanted to see Battler's reaction to what she had done. They will be with Shannon and Kanon too, and all will recognize that she "exists", but Battler won't because he said that every case of death could be explained in some ways (even if they were impossible XD). Beatrice was amused from him, and was happy to start a new game, trying to let him understand that she "exists" and that, behind all the murderers, there's her.
Everyone know that Beatrice is the Endless Witch and that she's very powerful and fickle, so, they try to don't talk about her, or she could get annoyed and make a trouble to the person who told her name (some examples are made, one of them is with Jessica's shoes that can't be found anywhere after that she nominated her). Instead, she was gracious to those who respected her.