Lahan is a civil official from the La Clan in The Apothecary Diaries who serves as the adopted son and biological nephew of Lakan, a strategist known for his eccentric brilliance. He is also the adoptive brother and biological cousin of Maomao, the protagonist. At nineteen years old, Lahan holds the position of royal treasurer, managing the imperial finances and overseeing the treasury ledger. His background is not fully detailed in the main story, but it is known that he was originally Lakan's nephew and was adopted into the La family, likely after being poorly treated in his original household. Lahan's personality is defined by an intense fixation on numbers and calculations; he sees the world through the lens of numerical data and carries an abacus in his sleeve at all times. Beneath an easygoing and polite facade, he possesses a dry, calculating mind and is capable of uncovering large-scale corruption and embezzlement solely by analyzing account books. Despite his shrewdness, he is socially reasonable and more emotionally expressive than his adoptive father, though he is notably oblivious to romantic interest directed at him and cannot handle alcohol at all. His primary motivation stems from his duty to Lakan and to the imperial court, using his numerical talent to investigate financial irregularities and political schemes. Lahan's role in the story involves assisting Jinshi by providing critical financial intelligence, such as tracing the flow of money to expose assassinations and embezzlement. Notably, he deduces Jinshi's true identity as a royal prince by comparing body measurements, and his first major appearance involves helping search for Maomao and the consort Loulan by bringing the treasury ledger to Jinshi. His key relationships include his bond with Lakan, for whom he often handles the aftermath of the strategist's chaotic actions; his connection with Maomao, whom he considers a younger sister and respects for her abilities; and his interactions with Jinshi, whom he negotiates with strategically, even requesting reduced repair costs for palace walls damaged by Lakan. He also has a biological brother named Junjie, though Junjie appears less frequently. Lahan's development is most apparent in his growing involvement with palace affairs and his willingness to use his financial expertise to support the central characters, while maintaining his own pragmatic and slightly cautious nature. His notable abilities include prodigious numerical skills, the capacity to spot minute discrepancies in ledgers, and a talent for uncovering fraud and hidden financial patterns that others miss.