## 1 Introduction  
During the last decade, technological advances greatly affected educational systems and have revealed novel tools and methods that are more attractive to students and have the potential to provide more efficient educational processes. Virtual reality constitutes an innovative learning technology that possesses great educational potential and is becoming an emerging technology in the educational landscape. Educational platforms that are based on virtual reality have the possibility to provide students and teachers with a wide spectrum of training procedures and have been used in many challenging fields and domains [1; 2; 3]. Virtual reality educational systems can be very assistive to both teachers in their teaching processes, and most of all, to students in their study, knowledge acquisition and understanding [4]. What is more, virtual reality has the potential to enhance the learning experience of the students and also improve the effectiveness, the attractiveness and the learning impact of the learning procedures [5; 6]. A crucial characteristic of 3D virtual reality platforms relates to the personal and social affordances that users experience. It is pointed out in many studies that an important issue of distant educational platforms concerns the lack of personal and social presence [7; 8]. In order to face this issue in virtual world educational platforms, embodied pedagogical agents can be utilized with the aim to enhance the social interaction with the learners. In educational environments that are based on 3D virtual worlds, pedagogical agents are mainly embodied avatars, which are employed to support students and scaffold the instructional aims [9]. The integration of embodied pedagogical agents in 3D virtual world educational systems can enhance their intelligence and believability, and most of all, improve the interaction with the students [10; 11]. The main impact of embodied pedagogical agents refers to the enrichment of the virtual learning environments with social aspects, better interaction with the users so as to support more adaptively the learning processes and enhance students learning experience [12]. In this spirit, pedagogical agents can have a central role in 3D virtual learning environments as they can serve diverse instructional goals and have a wide range of tasks and functionalities [13]. Embodied pedagogical agents are also utilized to scaffold self-learning by mimicking, to some degree, the role of the tutor. Such entities have also the ability to become learning companions as they can provide guidance and assistance to students during the learning process, [9]. Embodied pedagogical agents have also great promises for sophisticated, problem-solving assistance through the use of visualization techniques. Moreover, they can enhance students' learning with the delivery of individualized feedback, and in addition, can engage students in learning scenarios [14]. An area where the added-value of pedagogical agents is also identified concerns the development of the social and sensual presence (immersion), which leads students to feel and behave similarly to the way that the human-to-human interaction and communication occurs [15; 16; 17]. The proper design and integration of embodied pedagogical agents in virtual reality environments are necessary and highly desirable. The purpose of this article is to examine the learning effectiveness that agents have on students' learning experience, knowledge construction, and performance in 3D virtual world educational environments. In addition, we examine their role as learning companions and investigate the effect that they have on students' engagement as well as on their cooperation and communication. Furthermore, we examine how students perceive the role of pedagogical agents as learning companions during specific game-based activities and the effect that their guidance has on students' learning.