Nakai, a classmate of Nishikata and Takagi, stands out for his tall frame and perpetually closed eyes, a trait stemming from poor vision that persists into adulthood, where his height is complemented by a muscular build, stubble, and elongated sideburns. Known for his calm, easygoing nature, he exudes friendliness yet often seems oblivious, particularly around his girlfriend Mano. Despite this air of denseness, he quietly prioritizes their bond, such as discreetly inscribing her name in a book tied to a romantic legend—a gesture meant to honor their connection without public fanfare, initially misunderstood before its heartfelt intent came to light. Their relationship matures from navigating middle school trials, like dispelling waterslide rumors or coordinating festival dates, to cohabitation and eventual marriage, as shown in spin-off manga set over a decade later.

His friendship with Nishikata blossoms after admiring the latter’s earnest dash to school on their first day. The pair frequently assist each other’s romantic pursuits, with Nishikata aiding Nakai through shared tactics like an exchange diary. While Nakai’s relaxed approach to love contrasts with Nishikata’s flustered efforts, both invest deeply in their relationships.

As an adult, Nakai’s devotion to Mano endures through their shared life, though his occasional preference for male camaraderie sparks minor tensions. His tendency to sidestep overt romance—only to later unveil quietly sentimental acts—underscores a blend of nonchalance and sincerity.

His surname, blending the characters for "middle" and "well/mine shaft," holds no direct narrative significance. Notably, Nakai navigates daily life with his eyes nearly shut, relying on proximity to classroom boards to see clearly. Spin-off timelines estimate his age at 26–27, reflecting a ten-year gap since middle school.