Kazuki Yuiga is the younger twin brother of Hazuki Yuiga and the younger sibling of Nariyuki Yuiga, part of a family facing financial hardship. Their mother, Hanae Yuiga, supports the household through part-time work. Kazuki actively engages with his twin sister in attempts to match their older brother Nariyuki with various female acquaintances, demonstrating their shared interest in his romantic life.

He is depicted as a young child with short black hair and striking red or aubergine-colored eyes, a consistent visual portrayal across official media.

His given name "Kazuki" (和樹) combines the characters for "harmony, peace" (和) and "tree" (樹), while his surname "Yuiga" (唯我) translates to "solitary." His documented role remains centered on these established family interactions.