This paper introduces Interactive Virtual Persona (IVP), a multimodal LLM-based interactive user simulation that overcomes the limitations of personas, widely used to understand and communicate user needs in human-centered design. IVP is a multimodal LLM-based interactive user simulation that allows designers to conduct real-time interviews, share ideas, and obtain feedback via a voice interface. A qualitative study was conducted with eight UX designers, who used an IVP named "Alice" to conduct three design activities: user research, idea generation, and prototype evaluation. The results demonstrate the potential of IVP to accelerate information gathering, propose design solutions, and provide rapid user feedback.