In 2005, the capital and business alliance between Toyota and Subaru was announced. In 2007, the two companies began contract manufacturing of Toyota's Camry sedan on the production line at Subaru's U.S. plant, deepening their relationship in many areas, including production and development sites. And the Subaru Tresia is the result of this deepening relationship between the two companies. Although the Tresia was developed based on the Toyota Vitz , and is an OEM supply vehicle for the Toyota Lactis (2nd generation P120 model), in spite of more than 100 Subaru engineers participated in the development of the Ractis (2nd generation P120 model). Its development was carried out by engineers from both Toyota and Subaru. As a result, the RACTIS and TREZIA strongly reflect Subaru's car-making DNA and have a different driving feel from Toyota cars in the B segment up to now. The word "treasuria" was coined from the English word "Treasure" meaning "treasure.

The story is set in Tokyo, where an old school right in between the neighborhoods of Akihabara, Kanda and Jinbocho is in danger of consolidation and closure. Nine girls, centering Honoka Kosaka, have stepped up to defy the imminent danger facing their school. “If we want to protect our beloved school, we need to do what we can? we need to become idols!” Hoping against hope there’s something they can do about this, the girls strive to become famous, spread the word about their school, attract more students, and save the day. Now their “School Idol Project” begins to make their dreams come true! “Make our dreams come true!”

In charge of music production for μ's. A first-year student at Otonokizaka Academy. She is the only daughter of a wealthy family whose parents run a large hospital. While she has a hardworking and proud personality, she is not good at communicating with others. She had aspired to become a musician by writing her own songs, but she was about to give up her dream of becoming a musician because her parents expected her to take over the hospital as a doctor. At that time, Honoka Kousaka, the founder of the school idol group μ's, asked her to write a song for μ's and join as a member of μ's. After some hesitation, she agreed and became a member of μ's. She then followed her dream of becoming a musician again.

She is a second-year student at Otonokizaka Academy. She is the training coach and lyricist for μ's. She is a dignified, elegant, and gentle girl who has known Honoka and Kotori since childhood. Her family is the head of a Japanese dance school, and she has been practicing etiquette and martial arts since she was a child. She is also a hardworking person who loves to practice and balances her school idol activities with her archery club. She is a modest person by nature, and was initially reluctant to join the school idol club because of her fear of public appearances, but Honoka's enthusiastic solicitation persuaded her to join the school idol group μ's. Her special move is the "Love Arrow Shot," which shoots through the heart.