Guildenstern is an antagonist in the Onimusha series. He is considered the greatest scientist of the Genma world, and is responsible for the creation of many of the Genma soldiers that the heroes fight throughout the series. While he does not appear in person in Onimusha 2: Samurai's Destiny, he is referenced throughout the game in notes. He is also responsible for the creation of the Golden Evil Statue that Jubei Yagyu fought and destroyed, and for the creation of the Time Folder that is featured in Onimusha 3: Demon Siege.
Like other Genma found in the series, Guildenstern is named after a character from a Shakespeare play; in this case, the play Hamlet.