Pedagogical agents are animated avatars that stimulate cognitive and socio-emotive aspects of instructional activity in virtual learning environment. In the context of visual design, a pedagogical agent simulating human tutor can either be realistic (3D-rendered and naturalistic) or unrealistic (2D-rendered and stylized/cartoon). Though design factors such as agent stereotypes, aesthetics, roles and gender has been studied extensively, the issue of pedagogical agent's realism has not receive sufficient attention from the research community. This is unfortunate, as decision regarding agent's visual realism must be faced by instructional designers when implementing pedagogical agent in virtual learning environment. Hence, the objective of this study is to investigate the relationship among agent's visual realism, learners' cognition and learner's socio-emotive behaviours in a virtual learning environment. One hundred forty-four university sophomores at an Asian university interacted with either a realistic agent (3D-rendered and naturalistic) or an unrealistic agent (2D-rendered and stylized/cartoon) in a virtual learning system that delivers a lesson on basic computer programming. Results showed that the unrealistic (cartoonish) agent imposed extraneous cognitive load and hindered learning transfer, particularly for male learners. Mediation analysis demonstrated that the effects of agent's design (realistic versus cartoonish) on learning transfer among males were fully mediated by learner's extraneous load. The hypothesis that a cartoonish agent will elicit higher socio-emotive responses from learners was not supported. On the contrary, the realistic agent induced a higher level of positive affect in learners than the unrealistic agent Theoretical and design considerations regarding agents' visual realism are discussed in this paper.