Itsumi Momose is a young actress introduced during the Dark Moon story arc, where she plays the lead role of Mizuki Hongo, the love interest of Ren Tsuruga's character, Katsuki Tachibana. At the time of her debut, she is seventeen years old, born on November 16th, and is described as one of the new fast-rising stars in the Japanese entertainment industry. She has a slim build with long, pale blonde hair and grey eyes, and she later chooses to cut her hair short after the filming of Dark Moon concludes.

Personality-wise, Momose is depicted as a straightforward and professional individual. She is not afraid to speak her mind, as shown when she openly expresses disappointment in Ren Tsuruga's acting slump during a critical romantic scene, even going so far as to state that newcomer Kyoko Mogami was out-performing the established star at that moment. Despite her rising fame and innocent appearance, which has earned her a large male fanbase, she demonstrates a realistic and mature perspective on her work. When fellow cast members gossiped that she, as the romantic lead, should be the one closest to Ren Tsuruga, she sensibly pointed out that their characters are lovers on screen, which does not require them to be involved in real life. This grounded attitude reflects a deep sense of compassion and a strong professional ethic.

Her primary motivation as an actress evolves throughout the Dark Moon production. Initially excited to work with Ren Tsuruga, she becomes frustrated with his inability to perform. However, when the director and Lory Takarada test Ren, he improvises a scene with Momose that completely overwhelms her. She is left distraught after watching the playback, realizing that the performance he drew out of her was far better than anything she had previously achieved on her own. This experience transforms her outlook; she becomes determined to grow as an actor, using Ren as a goal to strive toward. Her commitment to her craft is further demonstrated when she requests to perform her own dangerous car stunts to make her acting more realistic, even though she dislikes thrill rides.

In the story, Momose serves as a catalyst for professional growth and a supportive peer. Her role opposite Ren highlights the pressures on set, while her interactions with Kyoko provide the newcomer with a genuine friend. Their relationship begins with initial suspicion, as Momose wondered if Kyoko used connections to get her role as the villainous Mio. This quickly changes as Momose is impressed by Kyoko's raw talent, and they develop a friendly rapport, eventually sharing a hotel room during a location shoot in Karuizawa. Momose also becomes perceptive to the off-screen dynamics involving Kyoko, noticing the strange tension between her and her childhood friend Sho Fuwa.

In terms of development and notable abilities, Momose is already a talented and popular actress, but her main arc involves refining that talent through intense challenge. She possesses the ability to hold her own in high-stakes dramatic scenes, but she learns that true greatness requires pushing beyond her limits. Her key relationships are primarily professional. With Ren Tsuruga, the dynamic shifts from co-star to a mentor-like rivalry, as she swears not to be overwhelmed by his acting again. With Kyoko Mogami, she evolves from a skeptical colleague into a close friend and roommate, often seen in her company alongside another actress, Airi Oohara. Throughout these interactions, Momose proves herself to be a professional actress dedicated to her work, grounded in reality, and a supportive figure to her peers.