Momoko lives in an apartment facing Saito's. A romance sparks between them after they observe each other through their windows. She harbors an intense, violent hatred for cockroaches, reacting immediately with force upon seeing one. This is shown when she ignores Naomi's plea, focused solely on crushing a roach underfoot, demonstrating determination and a violent nature. She wages war against a militaristic tribe of cockroaches in her own home, retaliating with insecticides against their food raids.

After moving into Saito's apartment, Momoko declares systematic war on the resident cockroach population. Her campaign escalates to deploying mass bug bombs and widespread insecticide spraying. She physically crushes roaches under her foot with such force the impact sounds like an earthquake, leaving dead insects beneath her bare feet. She also kills individual roaches, like crushing one feasting on leftovers under her hand.

Momoko's presence and relentless actions end the cockroaches' previously tolerated existence in Saito's apartment. Her relationship with Saito directly coincides with the shift from passive coexistence to active genocide against the insects. Her violence culminates in specific acts, such as dropping a book that kills the cockroach Hans during an escape attempt.