Tetsuko Koku is Punie Tanaka's classmate and best friend. Her name originates from "Kokutetsu," denoting the Japanese National Railways, combined with the feminine suffix "-ko," cementing her strong association with railway themes. She presides over the school's Railway Research Club, reflecting her dedicated enthusiasm for trains and railways.

Train-related visual motifs frequently punctuate her actions and scenes, often showcased through montages. Despite routinely facing dangerous situations orchestrated by Punie—typically as a sacrificial lamb for enemy traps—she remains innocently unaware of Punie's ruthless nature. This obliviousness persists even when directly encountering Punie's violence or serving as an unwitting instrument in schemes. Tetsuko displays a clumsy demeanor, reinforcing her role as a recurrent unintended victim.

She possesses red hair, red eyes, and wears glasses. Her standard school uniform commonly includes a ponytail and knee-high socks. No significant character development occurs, nor does her background story extend beyond her school role. She does not appear in spin-offs or additional media beyond the original OVA episodes. Her role remains consistently supportive, defined by her friendship with Punie, railway passion, and inadvertent involvement in hazardous scenarios.