Episode 12 of "Friendly Rivalry" unfolds with a bombshell revelation: Ye-ri's entanglement with Tae-jun comes to light, stunning everyone. In a heartfelt confession, Ye-ri unveils her family's financial ruin, driving her to desperation and forcing her into the confines of a goshiwon for weeks. Seul-gi, moved by her honesty, expresses gratitude for Ye-ri's transparency.
Meanwhile, the newly integrated Byeong-jin uncovers a crucial piece of the puzzle—Je-yi stands as the mastermind behind the drug market. His suspicions intensify when he learns that Su-jin, one of his associates, might have passed his fake IDs, used to procure prescription drugs, to Je-na.
Tragedy strikes as they discover Je-na's demise. Je-yi and Seul-gi rush to Seul-gi's father's hospital, where Je-na's battered body lies, a grim testament to a failed suicide attempt from a mountaintop, rendering her face unidentifiable. However, Seul-gi's keen eyes detect an odd detail: a yellow-painted fingernail and a distinctive foot scar, piquing her curiosity.
Sneaking into Je-na's hospital room, Byeong-jin stumbles upon his hidden cache of fake IDs, adding another layer to the mystery.
The aftermath of Je-na's death leaves Seul-gi visibly shaken. Conversely, Choi Gyeong finds herself on the cusp of a medical career, conditional upon maintaining her pristine image as a model student—a task complicated by the ongoing investigations entangling Je-yi and Seul-gi.
To uphold her pristine reputation, Choi Gyeong slips into the CCTV room, her actions caught by a vigilant Ye-ri, who watches her erase footage of herself breaking school rules after hours. Furthermore, a startling revelation unfolds: Choi Gyeong once witnessed Je-na in an intimate encounter with Seul-gi's father but chose to remain silent, keeping the secret buried.
Elsewhere, Je-yi is tormented by nightmares of her sister's demise. When Seul-gi visits her, she finds Je-yi in a fragile state, consumed by guilt and blaming herself for her sister's fate.
Driven by an unwavering determination to unveil the truth, Seul-gi embarks on a mission to trail Su-jin. Her mind drifts back to a haunting memory—an instance when she chanced upon Su-jin's distinctive foot scar, a scar born of bullying. Recalling the eerie similarity to the one she had seen on the presumed body of Je-na, Seul-gi is further propelled to validate her hypothesis. Seeking guidance, she encounters an acquaintance from Su-jin's rehab center, who leads her to Su-jin's goshiwon, a modest lodging establishment.
Parallel to Seul-gi's endeavors, Ye-ri and Choi Gyeong delve into the mysteries of their school's medical lab, where Je-na was last glimpsed during the school festival. This lab, intertwined with Tae-jun's medical center, harbors a pivotal clue—a scrap of paper bearing a goshiwon address, a room key, and a room number, each piece adding layers to the enigma.
Meanwhile, Seul-gi arrives at Su-jin's goshiwon only to be met with the devastating news of her demise. The owner, distraught and uncertain of how to proceed with the money Su-jin left behind, reveals the grim discovery. Seul-gi further learns that someone had antecedently settled Su-jin's rent and outstanding obligations, deepening the mystery.
Converging their paths once more, Seul-gi rejoins Ye-ri and Choi Gyeong, and the trio ventures into Je-na's room, which is replete with intricate solutions to mathematical puzzles, seemingly hinting at deeper, unresolved questions. The episode culminates in a tense moment as Seul-gi spots a locked diary, her curiosity overtaking caution as she forcefully opens it, teetering on the brink of uncovering long-buried secrets.
