Menthuthuyoupi, often called Youpi for short, is a character from the Chimera Ant arc. He serves as one of the three Royal Guards directly subordinate to King Meruem. Among the Royal Guards, Youpi is unique in that he possesses no human genetic material whatsoever; his composition is entirely derived from various magical beasts. He was born from the Chimera Ant Queen and named by her, and he has no memories or past-life identity inherited from any human components.

In terms of personality, Youpi initially presents as the most straightforward and simple-minded of the three Royal Guards. His lack of human genes means he cares little about his identity as an individual, leading him to focus on goals and act on instinct, which makes him a completely selfless and devoted soldier to the King. His nature is belligerent, vehement, capricious, and irascible, often to the point of confusion when confronted with the more eccentric behavior of his fellow guards, Shaiapouf and Neferpitou. When enraged, he tends to lash out indiscriminately and is prone to causing massive destruction without strategic thought.

However, Youpi undergoes a significant and rapid maturation process during his prolonged battle against the Hunters who invade the palace. As he fights Knuckle Bine, Shoot McMahon, Morel McCarthy, and later Killua Zoldyck, he begins to evolve. He learns to control his immense rage, using it as a strategic tool rather than being consumed by it. This evolution allows him to become a cunning strategist who can feign anger to lure enemies into traps. By the end of the battle, he reaches a state referred to as his Rage Incarnate form and, in a surprising turn, develops a level of respect and mercy for his opponents. He spares the lives of the defeated Hunters, thanking them for helping him grow stronger, a decision driven by a newly developed personal honor rather than blind obedience.

His primary motivation throughout the story is the absolute protection and service of King Meruem. This devotion is his defining characteristic, and all his actions stem from this singular purpose. He initially views humans and anything that threatens the King as obstacles to be eliminated. His relationship with the King evolves not as a personal bond but as a soldier's duty; after the King is mortally wounded by the Poor Man's Rose, Youpi selflessly donates most of his body's mass and life force to save him, resulting in himself being reduced to a tiny, weakened form. He does not share Shaiapouf's deep-seated hatred for Komugi, though he is perfectly willing to kill her if she becomes a burden to Meruem. After the King loses his memory, Youpi agrees to keep Komugi a secret to preserve the bond he and Pouf have with the amnesiac Meruem.

As a fighter, Youpi is an overwhelmingly powerful physical combatant. His immense aura总量 was estimated by Knuckle to be in the hundreds of thousands, dwarfing that of a professional Hunter. His primary ability is body transformation, which is linked to his Nen type. Sources classify him as either a Change user or an Enhancement user, but his signature power involves shaping his own flesh. He can freely morph his limbs into tentacles, sprout spikes, create additional eyes or limbs, and even grow wings to fly. By channeling his rage, he can exponentially increase his body size and aura output, unleashing devastating explosions of energy called Rage Blasts. His most powerful combat form is a towering, centaur-like creature that maximizes his destructive potential. After he sacrifices his body to revive Meruem, he is left in a much smaller, insect-like form with wings and a helmet-like head, but he is ultimately killed by the lingering radiation poison from the bomb that nearly killed his king.