A pedagogical agent is an anthropomorphic virtual character used in an online learning environment to serve instructional purposes. The design of pedagogical agents changes over time depending on the desired objectives for them. This article is a systematic review of the research from 2007 to 2017 related to the design factors of pedagogical agents and their impact on learning environments. The objective of this review is to identify and analyze pedagogical agents through the context in which they are constructed, the independent variables used in pedagogical agent research, and the impact of the pedagogical agent implementation. The review found that research on the design of pedagogical agents has different forms, namely text, voice, 2-D character, 3-D character, and human. The independent variables used in the studies are categorized into the appearance of agents and the role of agents. Moreover, the combination of pedagogical agent designs and role designs of pedagogical agents has significant positive impacts on student learning and student behavior. Recommendations are also provided at the end of this review.