Cooperative relationships between humans and agents are becoming more important for the social coexistence of anthropomorphic agents, including virtual agents and robots. One way to improve the relationship between humans and agents is for humans to empathize with the agents. Empathy can help humans become more accepting of agents. In this study, we focus on the IKEA effect in creating agents and investigate human empathy toward agents through relationships with others in the same space. For this reason, this study used a robot assembly task in which two participants cooperatively build the same robot or individually their own robot. We conducted experiments to examine the relationship between participants, the IKEA effect in creating an agent, and the influence of the empathy object on human empathy. The results showed that the IKEA effect promoted empathy toward the agent regardless of the relationship between participants. On the other hand, there was no significant difference in empathy from one participant to another before and after the task. These results indicate that regardless of the relationship between participants in the same space, the creation of an agent can promote empathy toward the agent.