Lahan is a civil official from the La clan and a supporting character in the story. He is the adopted son and biological nephew of Lakan, the head of the La family, making him the official heir to the clan. This also makes Lahan the biological cousin and adoptive brother of the protagonist, Maomao, a connection he emphasizes by referring to himself as her older brother. His biological brother, Junjie, appears less frequently. Physically, Lahan has narrow, fox-like eyes that are framed by round glasses, and his black hair is somewhat unkempt, falling just above his shoulders. In appearance, he has been noted as resembling a male version of Maomao.

At the young age of nineteen, Lahan serves as the manager of the imperial treasury, a position that grants him access to all financial records of the court. He is a numerical prodigy who carries an abacus in his sleeve and views the world primarily through numbers and calculations. He is exceptionally quick-witted and has an uncanny ability to spot discrepancies in ledgers that others would miss, as well as to connect those numerical anomalies to broader trends and conspiracies. For example, he once deduced Jinshi's true identity as a royal prince by observing the identical measurements of their bodies. His role in the story revolves around using his financial expertise to investigate fraud, embezzlement, and political corruption on behalf of his adoptive father Lakan and Jinshi.

Lahan is shrewd, observant, and cunning, much like the rest of his family. However, unlike his often unpredictable and socially detached adoptive father, Lahan is a more reasonable and socially adept person who can handle delicate negotiations and interactions with the court. He is more emotionally expressive than Lakan, often revealing his feelings of nervousness or frustration. Despite his brilliance with numbers, he is notably oblivious to romantic advances and does not notice the affections that several characters, including the apothecary Yao, hold for him. He is also a miser who runs several businesses, a trait that aligns with his calculating nature.

His motivations are strongly tied to his relationship with Lakan. Lahan deeply respects Lakan, who suffers from prosopagnosia and cannot recognize faces. To help his adoptive father identify him, Lahan deliberately wears distinctive glasses and always carries his abacus, using these visual and auditory cues as a personal identifier. This act demonstrates Lahan's deep desire for recognition and approval from the man he admires. While Maomao despises Lakan, she holds Lahan in high regard and acknowledges his abilities, recognizing that Lakan entrusts him with critical tasks. Their sibling dynamic is characterized by mutual respect, with Lahan acting as a self-proclaimed protective older brother. As the story progresses, Lahan becomes more deeply involved in the political and financial machinations of the imperial court, using his talents to uncover major conspiracies and assist Jinshi and Maomao in their investigations.