Keiko Yamamoto is a character from the 1991 anime OVA Judge, also known as Prosecutor of Darkness Judge. She is depicted as an adult woman with blonde or yellow hair and brown eyes. Her role within the story is rooted in her employment at a company where she works in the accounting department.

Yamamoto becomes entangled in a scheme involving two of her male colleagues, Ryuichi Murakami and Koji Kawamata. She is emotionally manipulated by Murakami, who seduces her to gain access to her position for the purpose of embezzling money from the company. Acting under the belief that she and Murakami would use the stolen funds to begin a new life together, Yamamoto agrees to participate in the embezzlement.

When an internal investigation into the company's missing funds begins, a rumor spreads identifying Yamamoto as the sole culprit. In this moment of crisis, Murakami betrays her, taking the embezzled money for himself and placing all the blame on Yamamoto. Overwhelmed by the betrayal and the prospect of facing the consequences for the crime she was manipulated into committing, she commits suicide by hanging. Her death is initially ruled a suicide by the police.

Yamamoto's primary function in the narrative is that of a catalyst. Her tragic death is the specific event that draws the attention of the protagonist, Hoichiro Oma, who secretly operates as the supernatural Judge of Darkness. Oma considers Murakami responsible for her death, treating it as a murder rather than a suicide, and appears before him to deliver a grim form of justice. Information regarding her personality, development throughout the story, or any notable abilities is not available.