"I'm Kinda Chubby and I'm Your Hero" centers on the life of Itsuki Midorikawa, a high school student who struggles with self-esteem due to his weight. Despite his insecurities, Itsuki possesses a strong sense of justice and a desire to help others. One day, he encounters a mysterious girl named Hana Sakurai, who is being harassed by a group of bullies. Itsuki intervenes, standing up to the bullies despite his fear, and this act of bravery catches Hana's attention.

Hana, who is cheerful and outgoing, becomes fascinated by Itsuki's courage and begins to form a bond with him. She encourages him to embrace his strengths and challenges him to step out of his comfort zone. As their friendship deepens, Itsuki starts to gain confidence in himself, though he continues to grapple with societal expectations and his own insecurities about his appearance.

The narrative explores themes of self-acceptance, personal growth, and the importance of inner strength over outward appearances. Itsuki's journey is marked by moments of vulnerability and triumph as he learns to value himself and his abilities. Hana serves as a catalyst for his transformation, but the story also emphasizes that Itsuki's growth comes from within.

Supporting characters, such as Itsuki's classmates and family members, add depth to the story by providing different perspectives on body image and self-worth. The manga balances lighthearted moments with more serious reflections on the challenges faced by individuals who do not conform to conventional standards of beauty.

Through Itsuki's experiences, the series highlights the impact of kindness, perseverance, and the power of believing in oneself. The story unfolds in a relatable and heartfelt manner, making it accessible to readers who may see themselves in Itsuki's struggles and triumphs.