Kaoru Tsutsui, younger brother of Hikari Tsutsui and son of Mitsuru and Kie Tsutsui, has dark blue hair and grey eyes. His personality is defined by disrespect and poor manners, including a consistent lack of sympathy for others—even his older brother. He rejects adult authority over his decisions and persistently insists he is right despite evidence to the contrary. This behavior surfaces when he tries to run away with his girlfriend, Anzu Takanashi, and defends his actions after her brother confronts him. His family never physically disciplined him, potentially contributing to these issues.

Kaoru shares a distant, strained relationship with his family, particularly Hikari. He avoids supporting his brother during hardships, prioritizing his own reputation over family loyalty and openly stating he does not wish to emulate Hikari. He participates in social events, such as visiting an aquarium with Anzu and attending Hikari’s wedding with their parents. His romantic involvement with Anzu Takanashi—younger sister of Hikari’s school rival, Mitsuya Takanashi—remains a notable aspect of his life.