What is the relationship between Gu Yan and Penglai?

The relationship between lovers Gu Yan and Penglai are lovers in the novel Long Night Sleeping. The novel tells the story of Gu Yan who goes to bed at night to be a night security guard in order to repay his kindness. In the story, Gu Yan and Penglai met and fell in love. However, Penglai's identity is not simple. She has something to do with paranormal in mysterious sleep. At the end of the novel, Gu Yan discovers Penglai's true identity as an emissary of the underworld, and exposes her plot. In the end, the relationship between Gu Yan and Penglai ended in failure.