one's first love is the 2nd chapter of the Citrus manga. It was released on March 18th, 2013.
Upon finding out Amemiya is using his engagement with Mei to become rich, and after she learns Mei not only knows, but doesn't care, Yuzu decides to take on the role of a big sisiter and reveals to the entire school body in their morning assembly that she witnessed Amemiya forcefully kissing a student on the school campus, without revealing Mei's identity. However, the ruckus infused by that action causes Mei to be taken away by her grandfather.
Yuzu, Mei, and their mother go out to shop. As they traverse the mall, Yuzu reflects over the true meaning of the kiss that Mei had surprised her with. After having a short conversation about Mei's family situation, Mei reveals that she hasn't seen her father in 5 years and that she'd been living with her grandfather, who is the school's chairman. Distracted by this new information, Yuzu trips into a fountain while dragging Mei down with her, forcing the family to go back home to bathe.
The next morning, Yuzu boldly greets the chairman in a familiar manner only to find that he does not recognize her as his granddaughter, while Mei is scolded for her lack of action. Later, sneaking back into the school because Yuzu refused to obey the chairman or to skip class, she and Harumi witness Amemiya talking to his lover over the cellphone, admitting to using Mei's hand in marriage only for financial gain and status.
At home, Yuzu finds that Mei is already aware of Amemiya's intentions, and Mei rejects Yuzu's advice. Later that night, Yuzu realizes Mei looks cuter when sleeping, and reaches out to kiss her, but is startled and apologizes when Mei calls her father in her sleep. The next day, Yuzu interrupts the morning assembly, and reveals that Amemiya had kissed a student at the school, not revealing Mei's identity, and requests that it's looked upon. When she arrives home after school, she finds her mother crying over a table. When Yuzu questions her mother, she tells her that Mei was taken away, which leaves Yuzu distraught.