The exact known time of when the Ostinato Musicians were founded is unknown, but they were established when Thorn, the group's leader, suggested to μ that she should make a group that would both protect her and compose songs for her whilst also acting as her own personal task force. μ agreed with this idea and chose six individuals from Mobius, each of whom was aware of Mobius' nature. Unlike the members of the Go-Home Club, they do not wish to return to the real world for various reasons, namely in part for μ promising to grant their utmost wish.
In truth, the Ostinato Musicians were meant to be used as tools by Thorn by filling μ with negative energy through their concert performances, which would make her more compliant to her wish to destroy reality.
In Caligula Effect Overdose, it's revealed that the Musicians have a secret room in between Mobius and the Metaverse-Es, made by μ as a hangout spot for the Musicians. The Protagonist is first introduced to this room and their new identity as Lucid should they choose to join the Ostinato Musicians.
- Thorn: The leader and founder of the Ostinato Musicians. As the leader of the group, she is the most dedicated to ensuring the Go-Home Club does not interfere with their agenda. μ trusts her explicitly and listens to whatever she says. She also bears a strong grudge against Shogo Satake.
- Sweet-P: A premier fashion icon who creates delicate, feminine-sounding music. She also has her very own fan group known as the Flower Princesses.
- Kagi-P: A former member of the Ostinato Musicians, following his defeat at the hands of the Go-Home Club.
- Shonen-Doll: A hooded young man who lives in Mobius' library, composing rather gloomy and depressing songs. He abhors the idea of companionship and despises anyone who leads a social life.
- Mirei: An immensely popular member who lives in luxury at the Sea Paraiso. She has many male followers and seems to have a one-sided rivalry with μ, though the latter is completely unaware of this.
- Ike-P: Izuru Minezawa's classmate, who is very narcissistic. He is popular with the female student body and enjoys showing off to further his own ego.
- Shadow Knife: μ's personal bodyguard and enforcer. He is tasked with hunting down traitors in Mobius under the guise of "justice." His appearance and mannerisms appear to be based on an anime character from the past.
- Wicked: A mysterious member of the musicians. Their gender is a mystery and no one knows much about them. They write violent, but impactful and deep songs.
- Lucid: The newest member of the Ostinato Musicians. They serve as the double agent of the Musicians, spying on the Go-Home Club.
- Kuchinashi: One of the Ostinato's more listless-looking members. She is one of the more social people of the group and surprisingly easy to talk to, and uses a special device to synthesize a voice rather than using her natural one.
- Stork: A good-natured Musician who's songs are very popular with the women of Mobius. However, due to his perverted tendencies, he is met with very little respect.
- Each Musician has a counterpart in the Go-Home Club with the exception of Lucid. The real life collaboration event with Princess Cafe listed the official pairs in the Menu as follows:
- Kagi-P and the Protagonist — Both have been allied with the Go-Home Club and the Musicians before (Overdose-only). Their contrast is expanded upon in the Caligula anime. Kagi-P is an untalented teenage boy who fears adulthood while Ritsu is a talented adult with socialization issues. Both had no desire to escape Mobius at first, but they became willing to confront their trauma due to their experience with the Go-Home Club.
- Sweet-P and Mifue — Sweet-P is a transgirl with a big appetite who is an obese man in the real world. On the other hand, Mifue has an extreme hatred for fat people that caused her to suffer from anorexia.
- Stork and Ayana — Stork is a voyeur who preys on young ladies. His criminal behavior disgusts Ayana, who has androphobia, and further exacerbates her condition.
- Shonen-Doll and Suzuna — They're both loners in high school. While Shonen-Doll fears social interaction to the point of becoming a shut-in, Suzuna still seeks friendship with others despite her alienation.
- Kuchinashi and Eiji — Kuchinashi is a victim of an arson that took her entire family's lives. She seeks revenge against Eiji, who she believes to be responsible for the crime. Eiji delights in their confrontations, but is visibly upset when he gets trapped by Kuchinashi or the Musicians - displaying the actions of a guilty man.
- Mirei and Kotono — The beauties of the two groups with a bitter rivalry. Beauty is considered a highlight in Mirei's life due to its association with a luxurious life. She seeks a return of her former riches. Meanwhile, Kotono's beauty costed her freedom and grounded her to the life of a single mother.
- Ike-P and Izuru — The handsome duo of Mobius. In reality, Ike-P is a typical-looking store clerk who is deeply insecure about his looks. His pursuance of fashion was in vain as it did nothing to ameliorate his situation. Izuru was born with good looks, but he finds it to be insignificant, if not a hindrance, to his life. Ike-P is envious of Izuru's physical appearance while the latter hopes to achieve independence like Ike-P.
- Shadow Knife and Kotaro — The two seekers of justice with a conflicting view on the method of approach. Shadow Knife demands retribution for his bullies and any who behaves similarly. He strictly disapproves of Kotaro's idealistic mindset of saving everybody.
- Wicked and Naruko — Wicked's abusive life warped her sense of compassion and destroyed her desire to connect with others. She finds relationships to be superficial and fragile. In contrast, Naruko requires constant affirmation from others to appease her loneliness.
- Thorn and Shogo — The loss of a shared friend traumatized the both of them. Unable to cope with the friend's death, Thorn wants to preserve her in the world of Mobius. This action has a huge impact on Shogo, who has been refusing to face his deceased friend and accept reality.
- Lucid — Lucid is alone as they are the double-crosser. As a double agent, Lucid is the only one that can form close bonds with both groups. They can make the decision to betray the Musicians or the Go-Home Club.
- "Ostinato" is a musical term referring to a portion of music that repeats constantly, befitting the Musicians' role in keeping Mobius' loops functioning.