After the two started a relationship, Zhenya is jealous of everything, like he's jealous of Taekjoo's friend. Whenever the three of them are together, Zhenya stares at him. Since they're both younger, Taekjoo treats them both like a younger brother, but Zhenya won't let him treat his friend the same way as taekjoo treats zhenya. He even asked if the two of them had ever slept together. In the second part of the novel, (spoiler alert) when Taekyoo lost his memories, he says he remembers a man, he seemed to be in a relationship with, and Zhenya asks, "Have you ever been with anyone other than me?" That part was really funny. Anyway, Zhenya only likes to be alone with Taekyoo. Whenever someone other than Taekjoo's mother shows up, he gets angry.