Koumi Natsuki serves as Urara Kasuga's best friend and key confidante. She energetically coaches Urara on romantic tactics to win Touma Inaba, often advocating direct or physical approaches inspired by Cosmopolitan magazine and her mother's view of men as providers. Her proactive nature compels her to actively involve herself in Urara's relationship struggles.

Natsuki initiates a sexual relationship with Tatsuhiko Mashu, a Keio University student and Touma's rival. This alliance leads her to aid Mashu in plots to sabotage Touma and Urara, notably supplying Touma with incense disguised as a memory aid but laced with addictive drugs per Mashu's scheme. Her loyalty to Mashu deepens into marriage aspirations and a desire to become a housewife.

After Urara's miscarriage and separation from Touma, Natsuki joins an escort service, recruiting both Urara and another girl, Mizuki, to hustle clients for money. This choice results in violent retribution: a client brutally beats Urara and orchestrates a gang rape against Natsuki.

Later, Natsuki undergoes a physical transformation, adopting a ganguro style with bleached blonde hair and a dark tan. She transitions into a career within the health and fashion industry, signaling a move away from her previous activities. Throughout these experiences, she remains Urara's closest friend, persistently offering support and engagement in her life despite surrounding turbulence.