Mercedes is shown to have brown hair worn in a bun, along with wearing a black dress with a yellow trim at the end herm with white sleeves.
Mercedes is visited by Cyrus after one of the latter's lectures to read The Church of the Sacred Flame: A Complete Historie, a rare and valuable tome. However, Cyrus realizes that the tome is missing from the library's archives. Mercedes then interrupts Cyrus' search to inform him that the Headmaster has summoned him to his office, much to Cyrus' indignation at the short notice. His indignation is only increased when it turns out the Headmaster summoned him primarily to scold him for citing a book in the archives during one of his lectures, which the Headmaster believes should be reserved for the Academy's private use.
Cyrus is quick to suspect that The Church of the Sacred Flame: A Complete Historie has been stolen, which surprises Mercedes as the keys are well-protected. Upon being Scrutinized, Mercedes tells him that only the Headmaster and the outside Guard have any keys to the archives. Cyrus soon concludes that the culprit is Russell, a fellow scholar of the Academy who has acquired a large gambling debt and stole the key from the Guard while he was sleeping, intending to sell the stolen tome for profit. Cyrus goes to confront Russell within his Subterranean Study and defeats him in battle, explaining that by stealing the tome, Russell would have deprived the people of knowledge that could be used for the benefit of society. A repentant Russell returns The Church of the Sacred Flame: A Complete Historie and turns himself in to the authorities.
Mercedes informs Cyrus that another tome called From the Far Reaches of Hell was stolen from the archives fifteen years ago, and that this tome is a spellbook containing ancient magic. Intrigued, Cyrus decides to travel Orsterra looking for clues as to where From the Far Reaches of Hell went, in the name of sharing more knowledge with others, unintentionally helped by one of his students, Therese when she started a rumor that Cyrus had entered an illicit relationship with Princess Mary, another one of his students, out of jealously that Cyrus was answering the Princess' questions in his lectures, which made Therese feel excluded.
Mercedes serves as the librarian in the Royal Academy of Atlasdam. As such, she has recurrent contact with Cyrus on a professional basis.
Mercedes was childhood friends with Zeph and Alfyn while she was living in Clearbrook, with the former having a crush on her. Approximately 10 years before the beginning of the game's current events, she moved to Atlasdam in pursuit of a larger life, upsetting Zeph and her when neither of them got the chance to say goodbye to each other. Ten years later, the protagonist can deliver unsent letters between the two, leading the two to meet once again and rekindle their relationship. Mercedes travels to Clearbrook and remains there upon completing the two "Zeph and Mercedes" side stories.