Dainari is a central character in the anime FLCL: Grunge, where he serves as the older brother of Shonari, one of the series' main protagonists. As a member of the Rockien species, Dainari possesses a body with rock-like skin and formidable natural strength, traits that define both his social struggles and his professional life. His name, along with his brother's, is derived from the Japanese mathematical symbols for less than and greater than, a detail confirmed by the series director Hitoshi Takekiyo.

Dainari's background is marked by tragedy and a heavy sense of responsibility. Five years before the main events of the story, following the death of his parents, a young Dainari takes on the solemn duty of disposing of their bodies in the ocean with the help of his then ten-year-old brother, Shonari. This act cements his role as the family's protector and provider. His personality is shaped by a harsh and pragmatic worldview, forged by the prejudice his family faces as aliens. When a young Shonari is arrested for severely injuring a bully in self-defense, Dainari confronts the police, arguing that his brother is still a child. When they refuse to accept this due to Shonari's immense size, Dainari consoles his brother not with soft words, but by dismissing the bully as weak and instructing Shonari to focus on his education. His ultimate goal is to secure a future for his brother away from their oppressive environment, urging him to save money and leave their planet.

Driven by this motivation, Dainari becomes an enforcer and cleaner for a local yakuza group known as the Keshibishi-gumi. His job involves brutal tasks such as delivering explosive packages to rival gang members and disposing of corpses, a role that the yakuza exploit due to his natural Rockien strength and resilience. This violent work takes a severe toll on his mental and physical state. He turns to alcohol to numb the psychological pain, and by the time Shonari is fifteen, Dainari is visibly suffering from the combined effects of alcoholism and brain damage sustained on the job. Despite his deteriorating condition, he continues his work, secretly hoarding his earnings in his pants to purchase a one-way ticket on the daily rocket shuttle for Shonari, demonstrating that his sole remaining driving force is his brother's escape.

Throughout the story, Dainari's most significant relationship is with Shonari. He acts as both a guardian and a tragic warning. As Shonari acts as his driver, Dainari becomes a broken figure, advising his brother not to wait until adulthood to plan his escape unless he wants to end up a drunkard like himself. His role in the plot culminates when a rival yaku gang war erupts. Despite being injured, Dainari attempts to protect the new leader of the Keshibishi-gumi. In the final confrontation, a rival Rockien enforcer plants an explosive into Dainari's already wounded midsection. Realizing he cannot survive, Dainari makes a final, selfless decision. He grapples the enemy, holding him in place so that they are both killed in the ensuing explosion, a sacrifice that saves the gang leader and removes a threat. His death leaves behind only his severed head, which the grieving gang leader holds as she vows revenge. Dainari's notable abilities stem directly from his Rockien heritage, granting him superhuman durability and physical power that make him a valuable, yet ultimately disposable, asset to the yakuza. His arc is a tragic one of a brother who destroys himself in a desperate, and ultimately fatal, attempt to buy a future for the only family he has left.