Kimio Nagasawa is a third-grade classmate of Maruko, distinguished by his uniquely onion-shaped head. He consistently exhibits a pessimistic outlook, frequently uttering offensive or awkward comments in group settings that provoke discomfort. Despite this negativity, he reveals a softer, more easygoing nature uncommon among his male peers.

Fujiki's friendship anchors Nagasawa emotionally, with positive reinforcement tempering his bleak episodes. Yet Nagasawa unpredictably rebuffs Fujiki at inopportune times, eliciting shock or disappointment. A pre-series house fire traumatized him, fueling pyrophobia and contributing to his pervasive pessimism.

He demonstrates profound concern for his frequently ill mother and has a baby brother, Tarou. Beyond Maruko and Fujiki, his social circle includes classmates Hanawa and Maruo.

His preferred foods are katsudon, sandwiches, and sea urchin, and he favors the band Zutorubi.

The spin-off "Nagasawa-kun" follows his high school years, retaining his signature pessimistic and ironic demeanor. This narrative delves into concealed personal struggles and family dramas absent from the original series. While masking emotional turmoil with defensive black humor, he navigates complex peer relationships and identity challenges.