<p>Responding to national calls to integrate generative AI into mathematics teacher education, this study developed Student ChatGPT, a custom chatbot designed as an interactive role-playing simulation for preservice secondary mathematics teachers (PSMTs). Specifically, Student GPT simulated a middle school student with misconceptions in ratio reasoning, providing opportunities for PSMTs for practice-based teaching experience. Through qualitative analysis of interactions between PSMTs and Student ChatGPT, an analytic framework was developed focusing on three domains: affective, communicative, and technical, to assess the behavior, strengths, and weaknesses of Student GPT. The results showed that the Student GPT demonstrates strengths in positive expression, clarity, relevance, error types, knowledge, and consistency. These advantages suggest that Student GPT can serve as a practice-based training tool to help PSMTs deepen their pedagogical content knowledge for teaching. However, limitations were noted in negative expression, language, acquisition, and role confusion. Despite these limitations, this study proposed that Student GPT is a valuable tool offering PSMTs low-risk, personalized, and interactive teaching experiences resembling real teacher-student interactions. Implications for mathematics teaching and directions for future research were discussed.</p>