1. A conversation-sentence generation device that generates a conversation sentence of a virtual agent having a personified conversation with each of a plurality of users, comprising: a memory that stores a physical and psychological state of the agent as an agent state and stores a physical and psychological state of each of the plurality of users respectively as a user state, the agent state including an emotion value representing an emotion of the agent, and the user state including a degree of intimacy value representing a degree of intimacy between each of the plurality of users and the agent; and a processor configured to execute instructions to implement: an input unit that receives, as input information, a conversation sentence given from one of the plurality of users to the agent, first clue information based on which the physical and psychological state of the agent is estimated, and second clue information based on which the physical and psychological state of the one of the plurality of users is estimated; an agent state estimating unit that estimates a new agent state based on the input information and the agent state, the agent state estimating unit further estimating the emotion value of the agent state; a user state estimating unit that estimates a new user state based on the input information and the user state; an utterance intention generating unit that determines whether said one of the plurality of users is an opponent user based on the input information, and that generates an utterance intention directed from the agent to the opponent user based on the input information, the agent state including the emotion value, and the user state; a conversation sentence generating unit that generates, based on the input information, the agent state, the user state of the opponent user including the degree of intimacy between the agent and the opponent user, and the utterance intention, a conversation sentence given from the agent to the opponent user; and an output unit that outputs the conversation sentence generated by the conversation sentence generating unit.