Hajime serves as the younger brother to Bakabon within their family. His name, meaning "beginning," signifies his arrival as the second son following an unexpectedly prolonged gestation. This delay triggered extraordinarily rapid mental development, bestowing advanced cognitive abilities upon him even in infancy. He demonstrates capacities for coherent speech, comprehension of complex subjects, and articulation of sophisticated concepts like the Pythagorean theorem shortly after birth.

His intellect embodies the "genius" referenced in the series title, forming a direct counterpoint to his brother Bakabon's lack of intelligence. This character design positions Hajime, alongside his mother, as the logical counterbalance within the family dynamic, offsetting the persistent foolishness of Bakabon and their father. Hajime's demonstrated abilities extend beyond theoretical knowledge to include multilingual comprehension, mastery of strategy games such as shogi, computer proficiency, and advanced reading skills.

Across all official media portrayals, including the *Rerere no Tensai Bakabon* adaptation, Hajime consistently retains his core characteristics: his status as a very young child possessing preternatural intelligence and the ability to communicate effectively despite his infancy. His primary narrative function remains providing a stark intellectual contrast to the absurdity and nonsensical behavior driving the series' comedic scenarios.