Kōji Tongari, nicknamed "Tongari," is a wealthy child from an affluent family. He displays an easily frightened nature, often crying and seeking comfort from his mother. Tongari exhibits extreme attachment to her, requesting permission for his actions and earning the label "mama's boy." His family owns a villa on the fictional Hyakujo Island, accessed by private plane during summer holidays. Academically, he ranks at the top of his class and attends nightly cram school. He struggles with pseudomyopia and had childhood incontinence, leading to the kindergarten nickname "Pee-in-the-piss Ton-chan." He dislikes green peppers and tomato juice.

Initially, Tongari acts antagonistically alongside his friend Butagorilla, bullying Korosuke—Kiteretsu's robot companion—through pranks like impersonating him with a ponytail hat during a temple fruit theft attempt. Both compete with Kiteretsu for Miyoko's attention. Tongari's character matures across the series, shifting from a boastful, cheeky figure influenced by wealth to a more sensible and reliable friend. This evolution includes distinct romantic interests, notably a crush on Hanamaru Satsuki, a theatre artist who briefly transfers to their school. Tongari's mother shows bias toward Satsuki during group visits, indicating awareness of his feelings. He eventually becomes a trusted companion to Kiteretsu, joining adventures and time-travel expeditions.

Beyond academics, Tongari excels in drawing and manga creation, winning a local children's art contest and receiving a mural commission for a public bathhouse. His manga submissions to the in-universe "Geragera Comic" magazine demonstrate exceptional ability, noted as unique among Fujiko F. Fujio's characters. He also masters toys like video games and mini 4WDs. Despite these strengths, he faces challenges including repeated runaway attempts to locations such as Nagano Prefecture and Izu. He secretly attends boxing gym sessions to relieve stress from Butagorilla's bullying but quits once discovered. Efforts to build confidence using Kiteretsu's inventions often backfire, resulting in failures. He once adopts a stray cat that saved his life after gaining his mother's approval. After high school, he enrolls at Takao University.

Tongari's relationships are complex. Though Butagorilla's best friend, he endures frequent bullying from him, prompting his secret boxing pursuits and occasional role as Butagorilla's "straight man." He acts as Kiteretsu's occasional foil, once harshly criticizing his inventions as unoriginal in a dream. Despite this, he becomes integral to the group dynamic. His family includes his father, Kōichi Tongari, and his mother, Takako, who influences his decisions. The character shares similarities with Suneo Honekawa from Doraemon and Haromi from Perman, reinforcing his archetype as a privileged yet emotionally dependent figure.