"Kyō no Kira-kun" centers on Niina Izumi, a reserved high school girl who struggles with social interactions and prefers to keep to herself. Her life takes an unexpected turn when she encounters Kira Yamada, a popular and outgoing boy in her class. Kira is known for his cheerful demeanor and ability to charm those around him, but he harbors a secret: he is terminally ill and has only a short time left to live. Despite his condition, Kira decides to live his remaining days to the fullest, and he becomes determined to bring Niina out of her shell.

The narrative explores the evolving relationship between Niina and Kira as they grow closer. Kira’s relentless optimism and zest for life begin to influence Niina, encouraging her to open up and engage with the world around her. Their bond deepens as they share experiences, from mundane school activities to more profound moments of vulnerability. Kira’s illness looms over their relationship, adding a layer of poignancy to their interactions, as both characters grapple with the inevitability of his condition.

Supporting characters, such as Niina’s classmates and Kira’s family, play significant roles in the story, providing additional perspectives on themes of friendship, love, and loss. The manga delves into the emotional struggles of those around Kira, highlighting how his presence impacts their lives. As the story progresses, Niina undergoes significant personal growth, transforming from a withdrawn individual to someone who learns to cherish her connections with others.

The plot balances moments of lightheartedness with emotional depth, creating a narrative that is both heartwarming and bittersweet. Kira’s determination to live authentically and Niina’s journey toward self-discovery form the core of the story, making "Kyō no Kira-kun" a poignant exploration of human relationships and the fleeting nature of life.