Rin Itoshiki is the youngest child and only daughter in the Itoshiki family, the fifth sibling among her brothers including Nozomu Itoshiki. Her horizontally written full name reads as "zetsurin" (絶倫), signifying "unequaled" in a positive sense, yet this term also carries slang connotations of exceptional sexual stamina, a meaning she finds profoundly offensive. This distress over her name fuels her intense desire to marry out of the Itoshiki household to adopt a spouse's surname and escape these associations.

Raised within the influential Itoshiki family of Kuraizawa, Shinshuu Prefecture, she embodies an aristocratic, traditional "Ojou" persona marked by refined manners and a sense of entitlement. A master of ikebana, she established her own school for the art, the Itoshiki School of Flower Arranging, where she serves as head teacher. Her family's considerable wealth allows her to wield financial power for personal convenience, exemplified by purchasing an entire neighborhood to avoid scrutiny from a classmate and subsequently evicting the residents.

Initially appearing as part of Nozomu's extended family, she later transfers into his class at the high school where he teaches. As a student, she occasionally breaks the fourth wall, deliberately turning away from the audience in a stylistic parody of anime tropes. Beyond ikebana, she displays diverse and improbably acquired skills, including ownership of a chocolate factory. Despite her outwardly gracious conduct, she exhibits manipulative tendencies, such as distributing golden tickets for chocolate specifically to socially isolated individuals to mock them. Her birthday is May 4.