Virtual Reality (VR) has recently attracted more attention in mental health applications due to its ability to immerse users in controlled and interactive environments. This paper presents a VR-Based AI Mental Health Companion, a multimodal system designed to support therapy, mindfulness, and real-time stress detection within immersive virtual reality (VR) environments. The system integrates artificial intelligence (AI) techniques by including natural language processing, emotion recognition, and physiological signal analysis by creating personalized mindfulness experiences and interactive meditation coaching. GPT-powered non-player characters (NPCs) are designed with specific therapeutic roles in mind, such as guided mindfulness facilitation, emotional support, and stress-aware conversational therapy. The system employs pose estimation to identify key body points and apply rule-based logic to assess posture accuracy during guided yoga exercises, providing real-time feedback to support correct movement execution. The work also includes biometric integration, such as EEG monitoring for enhanced emotional sensing. Expanding language support, increasing the diversity of pose datasets, and incorporating feedback from clinical professionals help refine the system. By combining immersive VR environments, GPT-driven therapeutic NPCs, and real-time posture validation, the proposed VR-Based AI Mental Health Companion demonstrates the potential of AI–VR convergence as a scalable approach to mental health care, with promising applications in stress management and preventative therapy.