Building Character for Conversational Agents: Ethos, Ethics, Credibility, and Believability[abstract] Because ethos is an unavoidable component of dialogue and forms the basis for believing and being persuaded by another's speech, it is an important topic for AI researchers. This paper examines the concept of ethos, especially Aristotle's notions of situated and invented ethos, as it functions in oral and written discourse and then explores what happens to ethos in computer-mediated human-to-human and human-to-machine discourse. The paper draws a number of conclusions that may be of value to researchers in these fields. In particular, it argues that the rhetorical concept of ethos furnishes a broader theoretical framework for understanding design and ethical issues involved in agent credibility than does the artistic notion of believability. The paper concludes by suggesting some nonartistic methods for making agents more credible within the framework of situated ethos.