Karurusu serves as prince of Honyaraland alongside his younger brother, Furanui. Sent to Earth by their father, King Kamopapa, to learn kingship through firsthand experience, Karurusu dons an outwardly optimistic persona dedicated to spreading happiness across Earth, framing his mission as a "tributary outreach program"—a direct contrast to Furanui's cynical focus on conquest through fear.

He wields shape-shifting abilities, transitioning between his natural form—a yellow-furred otter marked by a distinctive star on his stomach—and a human guise appearing as a blonde-haired young man with lilac eyes. His human attire features Arab-inspired clothing: a two-piece outfit revealing skin without exposing his abdomen, accessorized by a headband extending into a ponytail tipped with a star-shaped ornament.

Personality-wise, he exhibits childlike impulsiveness and frustration when plans falter, often clashing with Earth's customs due to cultural unfamiliarity. Despite initial pushiness, he genuinely strives to understand human emotions and societal norms, recognizing happiness requires effort rather than imposition. This drives him to develop methods like "Happy Powder," which erases traumatic monster-fighting memories while preserving contentment. His journey shows significant development: initially, he manipulates five Binan High students into becoming his "Happy Kiss" magic knights through forceful tactics, including magically binding resisters. Witnessing their willingness to risk their lives for his family during King Kamopapa's crisis deepens his respect for humans, reshaping his methods toward collaboration over coercion.

His powers extend beyond transformation to elemental magic like ice and fire, memory alteration, and interdimensional travel. He orchestrates the knights' transformations via a ritual where they kneel and kiss his hand, granting them celestial-themed weapons and flight capabilities. During conflicts instigated by Furanui's faction, he actively supports the knights, though he prefers resolving confrontations through emotional fulfillment over combat.

Karurusu's relationship with Furanui forms a core tension. Though older, he is not the crown prince; succession hinges on their father's evaluation of their kingly qualities. Despite this potential rivalry, he harbors no resentment, prioritizing familial bonds. When their father faces danger, his emotional breakdown reveals deep vulnerability and attachment. This moment also catalyzes his efforts toward sibling reconciliation, emphasizing his commitment to peaceful resolutions.