Kosaki Miyanagi is one of the main characters of the Nisekoi series. She is one of the love interests of Raku Ichijō.
Kosaki comes from a family of pastry chefs; all of whom specialize in making and decorating candies. However, her father is missing for the most part of the series and is hinted to have his own work to deal with. Thus, Kosaki has since been living with her eccentric mother and her feisty younger sister in their Japanese candy shop. Ironically, despite this environment, she grows up to be very timid by nature.

At one point during her childhood, Kosaki became acquainted with Raku Ichijō, Chitoge Kirisaki, Marika Tachibana, Yui Kanakura and Seishirō Tsugumi. They soon grew to be close friends with Raku in particular, whom she was in love with. In the day of their parting, the group proposes to give each other several keys and a locket (belonging to Raku) as a testament to their promise to meet again in the future, with the owner of the true key capable of unlocking the locket be the person Raku would marry. Kosaki would eventually receive the true key from Chitoge - whom Raku originally intended to be its recipient - after the latter overheard that the two are mutually in love and seeing their lover's mark carved on a rock. As such, Raku and Kosaki declared to each other their promise of marriage for the future.

As the memories of their childhood soon fade away, Kosaki has also gone to the same middle school as Ruri Miyamoto (Kosaki's best friend) and Raku.