Eva-Beatrice shows early on in this episode as another side of Eva Ushiromiya that acts as a kind of imaginary friend and dormant witch form that Eva developed during her youth. She makes her first appearance during a flashback to Eva's younger days to encourage her when she loses sight of her dreams and promises to turn Eva's anger and frustration into power to help her surpass her older brother Krauss and inherit the headship of the Ushiromiya family and all the wealth that comes with it.
Years later in 1986 during the Ushiromiya family conference on the island Rokkenjima a chance for them to do so appears. A letter is read up for the participants of the conference, telling them that if any one of them managed to solve the epitaph displayed under the portrait of the witch Beatrice and found a hidden gold treasure, the one that found it would be entitled to ten tons of gold and the family headship. With the help of her witch form, Eva manages to solve the riddle and finds the gold, gaining the headship in the process.
For figuring out the witch's epitaph there was also a prize in store for Eva-Beatrice as she's taken to the Golden Land shortly thereafter. She gains a body of her own as she becomes the successor to the Golden Witch Beatrice and inherits the titles of Golden and Endless from her along with the ability to use the Endless Magic, a form of magic that allows her to create, kill and revive with impunity.
After becoming a witch Eva-Beatrice instantly sets out to use her newly found powers to carry out a sacrificial ceremony to revive herself in an attempt to reach her full potential as a witch. She quickly finds that killing people is a thrill to her and would often revive her slain victims to kill them again in increasingly bizarre ways such as crushing them beneath gigantic cakes. She also shows a talent for summoning so-called Furniture such as Siesta 410 and Siesta 45 to do her bidding once she deems that the minions she inherited from her predecessor weren't "brutal enough" for her.
Ultimately her heinous acts puts her in conflict with her predecessor, the former Beatrice, which prompts Eva-Beatrice to attempt to murder her by having the Siestas pierce her heart with arrows and then by blowing her up when the arrows have no effect on her. This becomes the last straw for Battler Ushiromiya as he declares Eva-Beatrice to be his main opponent and challenges her to a Logic Battle. Eva-Beatrice almost wins this duel of wits as she manages to corner him, but is denied that victory as her predecessor uses the Red Truth to deny the existence of Witches, causing Eva-Beatrice to be vaporized.
Although she was essentially destroyed in the third episode, Eva-Beatrice makes appearances in later installments of the series. In Alliance of the Golden Witch she appears towards the end during an assault on Ange Ushiromiya where she spurs on Ange's attackers and in Dawn of the Golden Witch she appears as Eva Ushiromiya transforms into her during s scene with her son George Ushiromiya. She also appears in Twilight of the Golden Witch. Outside of the main series of visual novels, Eva-Beatrice has appeared as a playable character in every installment of the Ougon Musou Kyoku spin-off series of fighting games on the PC and XBox 360.