A virtual agent can have a graphical appearance and give users nonverbal information such as gestures and facial expressions. A computer system can construct intimate relationship with and credibility from users using virtual agents. Although related work have been discussing how to make users feel better about computers, a room for discussing effectiveness of letting users choose their favorite characters still exists. If a user's favorite agent is more believable than not favorite one for him/her, the computer system can construct better relationship with the user through an agent. In this paper, we have examined an effect of making user's favorite agent selectable on his/her behavior by conducting an experiment. We divided participants into four groups according to these conditions and ask them to have a conversation with an agent. As a result, we found a possibility of increasing credibility of an agent from users by letting them choose their favorite one.