Kai Xin, also called Zhen Kaixin in Chinese and Kaishin in Japanese, is Shi Fen's best friend. They attend the same high school and belong to a close group including Shi Fen, Wan Sui, and others. Kai Xin often joins Shi Fen in hatching schemes, like disguising as a couple to scam a buffet discount, leading to their ban.

Shi Fen's younger sister, Shi Miao, has romantic feelings for Kai Xin. This creates misunderstandings, especially when Kai Xin mediates between the siblings. He accidentally sparks confusion by telling Shi Fen "a cute girl" wants to meet him, unaware it's Shi Miao, resulting in a comedic confrontation. Later, Kai Xin congratulates Shi Miao on her perceived relationship with Wan Sui, ignorant of her feelings for him, which prompts her furious outburst toward Shi Fen.

Kai Xin occasionally displays obliviousness to Shi Miao's affections and social cues. After witnessing her outburst at Wan Sui and Shi Fen, he remarks on the complexity of "a girl’s heart," highlighting his lack of awareness. Despite this, he shows loyalty, like stepping in to swipe Shi Fen’s meal card for Shi Miao in the cafeteria when she lacks funds, an act deepening her affection. His role remains a supportive friend within the group, maintaining his steady presence throughout all seasons.