Hajime, also called Hajime-chan, is the younger brother of the main protagonist. His name means "beginning," signifying his position as the family's second child. Possessing extraordinary intellect from birth, he demonstrated advanced comprehension and verbal skills almost immediately, grasping complex subjects like Kepler's laws of planetary motion and the Pythagorean theorem as an infant.

His genius starkly contrasts the foolishness of his older brother and father, establishing him as the family's sole intellectual. This contrast forms an intentional dynamic, positioning him as a narrative foil to their antics. Despite his infancy, he engages in activities typically reserved for advanced age or education, including playing shogi, operating computers, extensive reading, and speaking multiple languages.

His established traits and role, consistently defined by his prodigious abilities from birth, remain unchanged across various anime adaptations, films, and spin-offs.