One of The American Roommate Experiment’s protagonists, Rosalyn “Rosie” Graham appears as a secondary character in Elena Armas’s previous novel, The Spanish Love Deception, as Catalina “Lina” Martín’s best friend and coworker at an engineering consulting firm. In The American Roommate Experiment, Rosie has left her corporate job due to finding success as a romance writer (under the pen name Rosalyn Sage). She finds this change daunting and difficult to explain to others, especially her father and younger brother, Olly. She is accustomed to “[keeping] a tight grip on any situation” (108), but writing is a departure because it is both less structured and “ruled by emotions” (108). Rosie’s father, a single parent, takes pride in her engineering career, making her terrified of disappointing him. This fear only intensifies when she struggles to write her second book, her first with a major publisher.

In addition to her professional upheaval, Rosie longs for romance, similar to the novels she loves and the relationship that Lina found with their coworker Aaron Blackford. Her dream of romance results in her interest in Lina’s cousin Lucas Martín, whom she follows on social media and expected to meet at Lina’s wedding. Lucas’s arrival in New York reignites this interest, with their first meeting comprising her mistaking him for a burglar.