This micro-ethnographic research focuses on a traditional custom on Toshi Island, in Japan. When first-born sons on the island graduate from junior high school, they form a small group of neya-ko (quasi-brothers) and sleep over at the house of their neya-oya (quasi-parents) every night until they reach the age of 26. They maintain the quasi-family relationship and help each other all their lives. Why does the neya custom still continue on this island, while most similar customs have already disappeared in other parts of Japan? To answer this question, we conducted participant observations and unstructured interviews. The results suggest that the ecological environment of the island has exerted an important influence on the neya custom. In spite of recent drastic social and economic changes in the islanders’ lives, the neya custom still plays a key role in building sustainability in the community. Based on these findings, we discuss how the multi-layered environments of the island interact with this specific custom that has been fostered through the years.