Kabuto Morino, the eldest of five Morino quintuplets by mere minutes, carries the English-dubbed name Karl Miller—preserving the initial of his Japanese given name. He habitually displays boastful tendencies and positions himself as a natural leader among his siblings, driving him to orchestrate matchmaking attempts and meddle in others' personal affairs during various adventures.

His childhood challenges include navigating a first romantic interest, forming a dedicated storyline, while moments of intense frustration occasionally surface—such as a runaway incident—revealing sensitivity beneath his confident exterior. Family dynamics, especially his perceived eldest status, compel him to spearhead group initiatives.

Interactions with his siblings mix collaboration and conflict, fueled by his assertiveness. He participates in family efforts like crafting secret gifts for their father but clashes due to his outspokenness. He immerses himself in school and community events, striving for academic excellence and pursuing record-breaking athletic feats.