Kazushi Sudō (須藤 和志) is a university student living in an apartment next door to the Shirakawa family home. He shares a deep, non-biological familial bond with sisters Ayane and Kotone Shirakawa, raised alongside them from early childhood. This history forged a sibling-like relationship where he holds genuine, sisterly affection for both.

He appears as a male teenager with brown eyes and short brown hair reaching his ears. His background notes he is a high school student.

Sudō exhibits occasional forgetfulness, such as failing to buy items Ayane requests. This trait directly triggers key events; once, forgetting led him to use Ayane's personal bath lotion without permission. Discovering this, Ayane retaliated by having Kotone join him during his bath, illustrating how his actions impact character dynamics.

The core of his background centers entirely on this lifelong, close-knit relationship with the Shirakawa sisters, defined by mutual affection and shared history despite lacking blood ties.