Watanabe likes Naoko, but Naoko has psychological problems and went to a nursing home, where Naoko met Reiko Kobayakawa, who is older than her. In the process of recuperation, Reiko Kobayakawa has always been Naoko's bosom elder sister. They sleep together every night and work and play together during the day. When there is no support in their hearts, Naoko becomes Ling He who supports each other. In other words, Reiko Kobayakawa and Naoko trust each other. Because of the relationship between Watanabe and Naoki, the appearance of Watanabe made Reiko Kobayakawa another trustworthy person. This is a good thing, because of this, Ling He Watanabe finally had sex.
Reiko Kobayakawa told Watanabe the news of Naoko's suicide. Reiko Kobayakawa itself has only two people to rely on, and that is Naoko and Watanabe. When Naoko died, Watanabe became a unique person, and Reiko Kobayakawa's feelings for him would naturally change. Compared with Watanabe, Reiko Kobayakawa has always been the middleman between him and Naoko. He and Naoko knew that Naoko's death also increased Watanabe's dependence on Reiko Kobayakawa. At the end of the novel, I think Ling He Junichi Watanabe is very willing to have sex, not because Reiko Kobayakawa and Junichi Watanabe have sex instead of Naoko, but because Ling He Junichi Watanabe has a certain degree of affection between them, and having sex is their comfort to each other. At the end of the novel, that background also prompted two people to have a relationship. At that time, Zi Ling came to say goodbye to Watanabe, which was also a way to say goodbye.
We in China may find it hard to understand why this is so. This is the difference between Japanese and China. Japanese people are sensitive and insecure, which is a powerful explanation.