While the complexity of 21st-century demands has promoted pedagogical approaches to foster complex competencies, a persistent gap remains between in-class learning activities and individualized learning or assessment practices. To address this gap, studies have explored the use of AI-generated characters in learning and assessment. One such attempt is scenario-based assessment (SBA), a technique that not only measures but also fosters the development of competencies throughout the assessment process. SBA introduces simulated agents to provide an authentic social-interactional context allowing for the assessment of competency-based constructs while mitigating the unpredictability of real-life interactions. Recent advancements in multimodal AI, such as text-to-video technology, allow these agents to be enhanced to AI-generated characters. This explanatory mixed-method study investigates how learners perceived the AI characters taking the role of mentor and teammates in an SBA mirroring the context of performing a collaborative science investigation. Specifically, we examined the Likert scale responses of 56 high school learners’ perception of the AI characters, in terms of trust, social presence, and effectiveness. We also analyzed the relationships between these factors and their impact on the intention to adopt AI characters for future learning through PLS-SEM. Our findings indicated that learners’ trust shaped their sense of social presence with the AI characters, enhancing perceived effectiveness. Additionally, the qualitative analysis further highlighted factors that foster trust, such as the credibility of materials and alignment with learning goals, as well as the pivotal role of social presence in creating a collaborative context. As an early study examining the role of AI characters in learning, this research contributes to the understanding of key factors influencing engagement with multimodal AI characters adding to the growing body of research on AI-assisted education.