Episode 13 of "Friendly Rivalry" kicks off with a poignant flashback, unfolding the intimate details of Je-na's life as penned in her diary, now inadvertently discovered by Seul-gi. Je-na had harbored a perpetual sense of inadequacy compared to Je-yi, driving her to extreme lengths to secure her father's affection, even crossing boundaries into an inappropriate liaison with him. This compromising position granted Tae-jun the upper hand to coerce Seul-gi's father into divulging the final math exam questions to Je-na.
In a meticulously orchestrated maneuver, Seul-gi's father, after the exam paper-setting session, mentally etched the most challenging questions, inscribed them onto a pill, and swallowed it, subsequently simulating illness to be transported to Tae-jun's clinic, where the pill was surreptitiously extracted through surgery. A startling revelation unfolds during this operation: Je-na was an unwelcome observer.
Switching back to the present, the mournful gathering for Je-na's funeral assembles, including Je-yi's estranged mother. Following the somber rites, Je-yi returns to bid her final farewells, where Seul-gi drops a bombshell, exposing Su-jin's and Je-na's duplicity. Je-yi, dread-laden, insists on shrouding Je-na's demise in secrecy, fearing her father's relentless pursuit of his nefarious schemes.
Seul-gi, fueled by rage, infiltrates Tae-jun's office and uncovers a vinyl record hiding an incriminating X-ray of her father. Driven to the brink, she lunges at Tae-jun, only to be restrained by her stepmother, who thwarts her impulsive actions. It transpires that her stepmother had steadfastly refused to妥协 the case.
Post-funeral, Seul-gi's sharp eyes catch Byeong-jin, confirming his role in settling Su-jin's debts. His presence at the funeral, motivated by Su-jin, serves as a covert clue that Je-na might still be breathing.
Seul-gi and her stepmother confer with their lawyer, Choi Gyeong's mother, who delivers a disheartening verdict: the vinyl record is legally inadmissible as evidence. Despite Seul-gi's earnest revelation of witnessing her father and Je-na together, Choi Gyeong's mother steadfastly declines to involve her son in the entanglement.
Elsewhere, Ye-ri plies her trade as a drug distributor, brokering a deal with Byeong-jin. Soon after, Byeong-jin entrusts Tae-jun with a recording, reminding him of a promise. As Seul-gi shadows them discreetly, an enigmatic arrival casts a shadow over the scene, bringing the episode to a cliffhanger conclusion.
