Virtual agents powered by artificial intelligence (AI) have been implemented in different service contexts, which have brought some changes to our lives. Previous studies have examined individual users' motivations to use virtual agents and the influences of virtual agents as social objects on individual users. There is a lack of knowledge on the relationship between humans and virtual agents, which could help understand the role of virtual agents in societies. In this work, we chose the mobile app Replika as our research context and utilized the big data analysis method to explore the major topics covered in online reviews about Replika on Twitter. Based on social penetration theory, we found four relationships between users and Replika, including relationship formation, exploration, maintenance, and destruction or termination. Our findings contribute to the literature by unrevealing a life circle of the relationship between human and virtual agents.