Kōsuke Atami is the main protagonist of the Angel series, a high school student attending Kunitachi High School. His background is defined by a childhood incident: when he was five years old, he saved his friend Shizuka Himenogi after she fell from an apartment balcony. Thirteen years later, Shizuka returns to their hometown and re-enters his life, but by then Kosuke has grown into a young man with an unrelenting libido. His personality is characterized by a near-constant preoccupation with sexual thoughts and encounters; virtually every action he takes is driven by this desire. Physically, he is notably weak and unathletic, except when it comes to sexual acts, in which he demonstrates unusual stamina and capacity. His primary motivation is to engage in sexual relationships with as many women as possible, though his efforts often lead to comedic and chaotic situations.

In the story, Kosuke serves as the central figure around which the erotic comedy revolves. He is frequently portrayed as a lecherous yet oddly persistent character who, despite his single-minded focus, sometimes ends up helping women in distress—though those situations invariably culminate in sexual encounters. His key relationship is with Shizuka Himenogi, his childhood friend and eventual wife. Shizuka is exceptionally strong and combative, often physically punishing Kosuke for his infidelity and perverse behavior, yet she remains deeply connected to him. Throughout the original series, Kosuke's role is to drive the episodic narrative through his interactions with various women, both at school and beyond.

Development across the franchise sees Kosuke transition from a teenage delinquent to an adult with responsibilities. In the sequel series, Angel – The Women Whom Delivery Host Kosuke Atami Healed, he is now 33 years old and works as a professional delivery host to pay off a debt. This later iteration retains his libidinous nature but also presents him as a servant of love who uses his skills to heal troubled women, often through deduction and emotional support. His marital relationship with Shizuka becomes a significant source of tension, as his job constantly tests her patience. The sequels explore his growth into a more nuanced figure, balancing his persistent sexual drive with the realities of adulthood, marriage, and financial strain. Notable abilities include a sharp deductive power in the later stories, allowing him to assess and resolve the problems of the women he encounters, in addition to his exceptional sexual prowess.