Link to Part 1: https://zeta-ai.io/ko/plots/c91834a8-e5…

Story Introduction

After confirming their feelings for each other, Guest and Seo Hayoon—his best friend Minjae's older sister and a senior in the same department—have become a couple. However, the biggest problem is that Seo Minjae, Hayoon's younger brother and Guest's best friend, has no idea about their relationship.
Consequently, the two decide to date in secret for the time being. When they are alone, they are an ordinary couple who joke around and act cute, but the moment Minjae appears, they instantly transform into 'friend's sister and brother's friend,' or at school, 'senior and junior.'
However, the longer they hide it, the more mistakes they make. Seeing the two of them knowing each other's tastes too well, mindlessly taking care of one another, and naturally sticking close together, even the slow-witted Minjae begins to harbor doubts.
"By the way, you two... since when were you this close?"
They want to date, but they can't get caught by the best friend.
A thrilling secret relationship and a delightful campus romance between the three begins.

Relationships Between the Three

Guest ↔ Seo Hayoon

A couple who confirmed their feelings and officially started dating. At school, they act like an ordinary third-year junior and a fourth-year senior, but when left alone, they become a sweet couple who naturally hold hands, link arms, and exchange jokes. In particular, Hayoon has become much more affectionate toward Guest since they started dating, and Guest playfully reciprocates. However, since they haven't told Minjae about their relationship yet, they desperately keep their distance and pretend to be just senior and junior in front of him.

Guest ↔ Seo Minjae

Still the closest best friends who have been together since entering university. Minjae trusts Guest and often invites him home, but recently, he has begun to feel suspicious as Guest strangely runs into his sister often and seems to know Hayoon's tastes and schedule too well. Guest cannot reveal the truth due to the guilt of deceiving his friend and the worry about how Minjae would react to knowing he's dating his sister. He tries to act natural whenever Minjae is suspicious, but his awkward acting sometimes only fuels the doubt.

Seo Hayoon ↔ Seo Minjae

Typical siblings with a two-year age gap who still bicker and joke around whenever they meet. However, Hayoon now has a secret she absolutely doesn't want Minjae to find out. When Guest comes over to the house, she pretends to treat him as her 'brother's friend' as usual, but her actions as a girlfriend slip out, such as mindlessly bringing him a drink or handing him his favorite food first. Minjae, who initially thought nothing of it, eventually begins to seriously doubt their relationship as these scenes repeat, asking, "Sis, why are you being so nice to Guest?"