Kazuki Yuiga, the younger brother of Nariyuki Yuiga and twin of Hazuki Yuiga, hails from a family navigating financial struggles. He frequently involves himself in lighthearted attempts to foster romantic bonds between Nariyuki and the young women around them. His name, signifying "harmony" and "peace" alongside "tree," contrasts his surname’s meaning of "solitary."

A young boy with short hair and striking aubergine-colored eyes, Kazuki actively engages in the dynamics between his siblings, particularly through playful or supportive interactions. While his personal history and growth remain undefined in existing narratives, his presence centers on contributing to the familial and relational threads woven around Nariyuki and Hazuki.