Kippei Katakara is the main protagonist of Aishiteruze Baby, introduced as a seventeen-year-old high school student living a carefree and self-centered life. He is described as an easygoing and well-known playboy, spending his days flirting with girls, sleeping on the school roof, and generally avoiding responsibility. His friendly and uncomplicated nature makes him extremely popular with his female classmates, and he takes full advantage of this attention, often without considering the emotional consequences for others. This initial characterization presents him as a superficial slacker who seems to have little direction or serious ambition.

The central conflict of the story begins when his five-year-old cousin, Yuzuyu Sakashita, is suddenly brought to live with the Katakara family. Yuzuyu has been effectively abandoned by her mother, who is unable to cope with the stress of being a single parent following her husband's death. While Kippei's mother and sister feel great sympathy for the child, they are too busy to take on full responsibility, and the task of her daily care is unexpectedly assigned to Kippei. This responsibility forces him to drastically alter his lifestyle; his former pastimes of chasing girls and sleeping in are replaced with the duties of waking early to make Yuzuyu's lunch, walking her to kindergarten, and ensuring her overall well-being. Initially, he is completely out of his depth, and his early attempts at childcare are clumsy, such as making an extra-large rice ball for her lunch, which Yuzuyu nonetheless eats because Kippei prepared it for her.

Kippei's primary motivation evolves throughout the story from avoiding work to becoming a devoted and nurturing caretaker. He grows deeply attached to Yuzuyu and becomes highly attuned to her psychological needs, particularly her feelings of loneliness and grief stemming from her mother's absence. This relationship is the catalyst for his character development; through the act of caring for a vulnerable child, he sheds his promiscuous and irresponsible ways, maturing quickly into a responsible and dedicated parental figure. His role in the story is that of a fish out of water, a teenager forced to learn the fundamentals of childcare and household management, with his personal growth serving as the emotional heart of the narrative.

Key relationships are central to Kippei's development. His bond with Yuzuyu is the most significant, transforming him from a boy into a nurturing guardian who prioritizes her happiness above his own. He also pursues a serious romantic relationship with a quiet and lonely classmate named Kokoro Tokunaga. Unlike his previous shallow flings, his relationship with Kokoro is built on mutual understanding and complementing each other's personalities; his carefree attitude helps lighten her emotional burdens, which stem from her mother's death and her father's remarriage. His family, including his grandmother, mother, and older sister, provide a support system and living environment, but Kippei remains the primary person responsible for Yuzuyu's day-to-day upbringing. A notable ability that Kippei develops over the course of the series is cooking. He progresses from being unable to make a decent meal to becoming skilled enough to consistently prepare lunches and dinners for Yuzuyu, a practical skill that signifies his growing competence as a caregiver.