Embodied conversational agents have changed the ways we can interact with machines. However, these systems often do not meet users' expectations. A limitation is that the agents are monotonic in behavior and do not adapt to an interlocutor. We present SIVA (a Socially Intelligent Virtual Agent), an expressive, embodied conversational agent that can recognize human behavior during open-ended conversations and automatically align its responses to the conversational and expressive style of the other party. SIVA leverages multimodal inputs to produce rich and perceptually valid responses (lip syncing and facial expressions) during the conversation. We conducted a user study (N=30) in which participants rated SIVA as being more empathetic and believable than the control (agent without style matching). Based on almost 10 hours of interaction, participants who preferred interpersonal involvement evaluated SIVA as significantly more animate than the participants who valued consideration and independence.