Konoe Tsuruma is a central figure in the narrative, a student at Miryuin Private Academy who becomes the protagonist's first friend and classmate. She serves as the class representative, a testament to her diligent and responsible nature. Her background is intertwined with technology, as her father is a leading designer of electronic devices whose inventions occasionally play a role in her actions. A foundational part of her identity is her connection to the protagonist, Shougo Mikadono, as a childhood friend. She carries deep guilt over a childhood accident that left him with a scar, which motivates much of her interaction with him. The most defining element of her background is her true identity: in the resolution of the source material, she is revealed to be Shougo's half-sister, the biological daughter of his father from another relationship, while Shougo himself was conceived through artificial insemination, placing them in a complex sibling relationship without a direct blood bond.

Konoe possesses a soft and gentle appearance, with long black hair and olive eyes, which matches her polite and inquisitive personality. She is characterized by a strong belief in destined encounters, a trait that deeply influences her romantic outlook and her interpretation of her relationship with Shougo. She also has a fondness for cream puffs, a simple detail that marks the beginning of their connection. Her primary motivation stems from a childhood desire to be Shougo's sister so they could remain together forever. This wish evolves into genuine romantic feelings, creating a profound inner conflict when the possibility arises that she might actually be his sibling. Throughout the story, she is driven by a protective loyalty, a sense of duty, and a hope to reconcile her past actions with her present feelings.

Her relationship with Shougo is the cornerstone of her character arc, evolving from a casual friendship into a romantic entanglement complicated by shared history and family secrets. This dynamic forces significant personal development upon her. She transitions from a passive character waiting for destiny to an active one who confronts her guilt and seeks to define her future alongside Shougo. Her role in the story is multifaceted: she is a primary romantic interest, a key suspect in the overarching mystery of the protagonist's hidden sibling, and a stabilizing presence in his new school life. Her interactions with the other female students are often competitive yet cordial, as they are simultaneously romantic rivals and suspects in the same mystery, a position that heavily shapes her social role. Konoe's notable abilities are not supernatural but rather a combination of resourcefulness, demonstrated through her use of her father's technology, and a powerful emotional resilience. Her steadfast loyalty and capacity for self-reflection enable her to navigate the complex emotional and ethical turmoil that defines her personal mystery and ultimate resolution.