Jade Leech is a second-year student at Night Raven College and serves as the vice-warden of Octavinelle, the dormitory inspired by the sea witch Ursula. He is the twin brother of Floyd Leech and acts as the primary assistant to the dorm's warden, Azul Ashengrotto. Hailing from the Coral Sea, where he, his brother, and Azul have been close since childhood, Jade is a moray eel merperson who has adapted to life on land. His appearance is distinguished by his great height, teal hair with a distinctive longer dark grey strand, and heterochromatic eyes, with his left eye being golden-yellow and his right eye olive-green. He wears a blue earring made from sturgeon scales on his left ear, a traditional lucky charm from his underwater home, matching the one his twin wears on the right. True to his formal personality, he keeps his Octavinelle uniform impeccably neat, with his black fedora, double-breasted suit, and purple dress shirt all arranged with precision.

In stark contrast to his brother's volatile and impulsive nature, Jade presents himself with a calm, gentlemanly, and unfailingly polite demeanor. He always speaks with a smile and uses formal, respectful language known as keigo, which has led to him being voted as the character most associated with polite speech. However, this polished exterior masks a deeply unsettling and sadistic core. He can discuss dark or disturbing subjects with the same pleasant expression, revealing a tendency to derive enjoyment from the suffering of others, particularly when he is the cause. While his politeness might seem like a calculated tool to maintain distance or observe others without giving away his true intentions, evidence suggests this manner of speaking is a deeply ingrained part of his personality, possibly learned from his mother, rather than a purely manipulative act. He often makes sharp, sarcastic, or outright rude remarks delivered in the most courteous of tones, a trait that becomes more pronounced when he teases his close associates, especially Azul.

As the vice-warden, Jade acts as a supervisor for both Azul and Floyd, monitoring their actions and stepping in to manage situations when necessary. He is more obedient to Azul than his twin is, yet paradoxically is also more likely to take subtle jabs at him. His motivations appear rooted in a love of observation and control. He finds immense entertainment in watching the reactions of others, whether they are the targets of the dorm's manipulative schemes or his own friends. He is the founder of the Mountain Lovers Club and has a particular fondness for collecting mushrooms and creating terrariums, hobbies that reflect his meticulous and patient nature. Unlike Azul, who is a determined and hardworking schemer, or Floyd, who is an unfettered natural force, Jade operates within established frameworks, skillfully manipulating them to his own amusement. For all his formal exterior, he is still a seventeen-year-old boy who can be childish and petty among those he trusts, revealing a more genuine and less guarded side only when in the company of his twin and Azul.

Jade’s role in the story is primarily as a key enforcer of Azul’s contracts and schemes, using his intellect and unsettling presence to pressure opponents. He is fiercely loyal to the inner circle of Octavinelle, his bond with Floyd being almost symbiotic, while his relationship with Azul is built on a foundation of mutual trust, deep history, and a shared, if unequal, sense of rivalry and respect. His most notable ability is his unique magic, called Shock the Heart. This spell forces anyone who makes eye contact with his left eye to answer his next question truthfully. The spell has severe limitations: it can only be used on a single person once in their lifetime, as he discovered by testing it on his brother, and it is ineffective against powerful mages or particularly cautious individuals. It is most potent against those who are off-guard, frightened, or emotionally vulnerable, requiring Jade to apply psychological pressure before he can employ it successfully. He prefers to keep this ability a secret, as its nature would make people wary of him. Like all merpeople in the story, he also possesses the ability to transform back into his mermaid form when submerged in water.