In a lecture, it is essential to stimulate learners’ interest in the lecture content, which brings about their engagement. However, it would be challenging. In this work, we focus on personality traits that teachers express throughout their lecture. Their non-verbal behavior including gaze, gesture, facial expression, and personal episodes related to the lecture content often expresses their personality traits. Learners are expected to be interested in them, which drives their interest in the lecture content. To express different personality traits for different learners, we focused on a social robot as a teacher. In this paper, we introduce the design of personality traits for a robot used in a lecture. We also demonstrate a robot lecture system in which a teacher robot expresses specific personality traits designed in advance to conduct a lecture. The results of a case study with the system suggested that the teacher robot had the potential to improve learners’ engagement.