Sai is a 12-year-old girl working as an assistant to the Master Mapmaker in the kingdom of Mangkon. Though she pretends to come from a respectable family, she actually lives in poverty in a swampy area known as the Fens. She possesses a remarkable talent for forgery and is desperate to escape her current circumstances. Seeking a better life and a chance to distance herself from her background, she joins a royal expedition to map new territories.

Rian is an upper-class young woman traveling on the royal expedition. She holds the Lineal of Honor for her bravery in the recent war. Despite her high status, she displays an easy familiarity with the crew and takes a quick liking to Sai. She possesses a deep ambition to achieve glory and honor through discovery.

Paiyoon is Mangkon's Master Mapmaker, known for his exquisite hand-drawn charts and old-fashioned techniques. Despite his gruff exterior and frequent complaints, he treats his assistant fairly. Old age and arthritis cause his hands to tremble, leading him to rely heavily on Sai's skills for the upcoming royal expedition. He carries a complex history tied to the kingdom's past expansion.

Mud is Sai's father, living in a one-room apartment in the swampy area known as the Fens. He struggles with alcohol addiction and makes his living running confidence schemes. Though he insists he wants to earn enough to give his daughter a better life, his actions consistently force her into difficult positions. He relies on her forgery skills for his illegal activities.

Captain Sangra is a revered war hero and the great-niece of Mangkon's Queen. Known for her bravery and military successes, she commands the Prosperity on its southbound expedition. She maintains strict discipline on her ship but also demonstrates battle-weariness, preferring to avoid unnecessary risks. She is widely respected by both her crew and the kingdom at large.

Bo is a young boy from Pitaya Island who resorts to petty theft to survive. He stows away on the Prosperity in hopes of reaching An Lung to start a new life. He carries a hardened exterior and a quick temper, but he also possesses a strong desire to find legitimate work and escape poverty.

Grebe is a young sailor on the Prosperity with a scarred face. He originates from the Fens in An Lung, sharing the same low-class background as the protagonist. His presence on the ship creates immediate tension, as he possesses the knowledge to expose Sai's true origins to the rest of the crew.

Mr. Lark is the Prosperity's resident naturalist. He participates in the expedition to collect plant and animal specimens for the Queen's new museums. He views the natural world primarily through the lens of ownership and discovery, assuming the right to claim and rename the creatures he encounters.

Dr. Pinching serves as the surgeon aboard the Prosperity. Like the naturalist, she wears formal, upper-class attire that stands out among the working sailors. She tends to the crew's medical needs during the long voyage across the ocean.

Catfish is a confidence man operating in the Fens of An Lung. He spends his time at local taverns plotting illegal schemes. He frequently attempts to convince others to assist in his forgeries to ensure his plans succeed.

Tippy is a young Assistant working in An Lung. She comes from a respectable, upper-class family, a status she proudly displays through her gold lineal. She embodies the privileged society that dictates the kingdom's rigid social hierarchy.