Guen serves as the sole court physician assigned to the inner palace, a position he fills despite a profound lack of formal medical training. His physical appearance is that of a plump, middle-aged man with black hair and small eyes, often recognized initially by a distinctive, thin mustache that Maomao feels resembles that of a catfish; this mustache is later shaved off by order. As a eunuch, Guen has a notably effeminate voice and manner, and he has channeled his lost libido into a great love of eating and small comforts.

Before entering the palace, Guen came from a humble background as a paper maker in a village that fell on hard times. When logging restrictions under the Empress Regnant crippled the family business, he faced a terrible choice. To prevent his younger sister from being sold into palace service as a court lady, Guen volunteered to be castrated and become a eunuch in her stead. He entered the inner palace as a physician, not out of ambition or a calling to medicine, but as an act of profound self-sacrifice to protect his family. His inability to perform the duties of a doctor stems directly from this lack of real training; the palace physicians who attempted to educate him gave up after witnessing his complete ineptitude, famously failing to even outwit a chicken during a dissection exercise. Because qualified physicians are unwilling to become eunuchs, however, Guen remains in his post as the only medical officer officially allowed inside the women's quarters.

In personality, Guen is fundamentally kind, friendly, and good-natured, though these traits are coupled with a meek and insecure disposition. He is guileless and somewhat loose-lipped, often revealing information without realizing its political significance. His most defining characteristic is his complete absence of medical competence. He is terrified of corpses, has only a rudimentary grasp of toxicology and diagnosis, and is incapable of performing an autopsy or managing the medical stores effectively. Because of this, almost no one in the inner palace relies on him for actual medical care. His nickname, the Quack Doctor, is used internally by Maomao and reflects his reputation.

Guen's role in the story is deeply intertwined with Maomao. Initially, he is wary of her as an intruder in his office, but he quickly recognizes that she is far more capable. He discovers that delegating difficult tasks to her makes his own life easier and yields better results. With the unofficial permission of Jinshi, Guen allows Maomao to use his medical office, shares his workload with her, and effectively serves as her physical proxy, as a woman cannot hold the official title of court physician. He becomes very fond of Maomao, showing his hospitality by thoughtfully providing her with tea, sweets, and access to medicinal ingredients. Instead of showing jealousy toward her superior skills, he is appreciative and relieved, and he demonstrates a similar admiration and respect for other skilled individuals like the apothecary Luomen.

Beyond his relationship with Maomao, Guen maintains a connection to his past through his knowledge of paper-making, an ability he occasionally employs. Despite his comic role and constant mishaps, Guen is defined by his quiet heroism and resilience. He does not resent his position or the world that mocks him, but instead carries on with a gentle smile, supporting those around him without seeking praise. His background as a man who sacrificed his entire future for his sister provides a poignant counterpoint to his slapstick incompetence, making him a warm, imperfect, and deeply sympathetic figure within the cold political machinery of the palace.