Tetsuko Koku serves as both classmate and self-proclaimed best friend to protagonist Punie Tanaka. She holds the presidency of the school's Railway Research Club, embodying her avid railfan identity. Her name directly references Japan National Railways (Kokutetsu) with the feminine suffix "-ko," a connection reinforced by recurring train imagery during her scenes, often presented in montages.

Tetsuko exhibits a consistently naive and trusting personality, remaining innocently oblivious to Punie's violent tendencies despite frequently suffering collateral damage. She functions narratively as the "Straight Man," reacting with horror and confusion to surrounding absurdities while maintaining emotional sensitivity. Visually, she features red hair styled in a ponytail, red eyes, glasses, and a school uniform with knee-high socks.

Throughout the series, Tetsuko endures repeated physical exploitation. She becomes a sacrificial lamb for traps set by Punie's enemies, serves as a human shield or distraction during assassination attempts, and sustains injuries from magical attacks. She also faces verbal abuse, harsh criticism, and manipulation by Punie. Despite this mistreatment, she maintains unwavering loyalty, continuing to follow Punie even after witnessing her brutality.

Tetsuko develops a tentative friendship with Anego, leader of a girl gang, assisting in arranging a date between Anego and classmate Yamada. This effort fails due to Punie's interference. Her role remains defined by her railway obsession, victim status, and function as a contrasting figure to the main cast's violence.