Persistent inequities in STEM education underscore the need for teachers who can integrate rigorous content instruction with equity-focused, socially and emotionally supportive practices. This study examined how preservice secondary STEM teachers conceptualize their emerging professional identities, beliefs about social and emotional learning (SEL), and the relationship between SEL and equity in shaping their pedagogy. Using a phenomenological approach, we conducted semi-structured interviews with eight student teachers enrolled in an undergraduate credential program in the Western United States. Interviews were analyzed through a combination of deductive coding informed by the transformative SEL (T-SEL) framework and inductive open coding to capture emergent themes. Findings revealed that participants defined their professional identities as facilitators and advocates for equity, while navigating tensions between relational commitments and systemic expectations that privilege content delivery. Teachers consistently articulated SEL as essential to fostering engagement, resilience, and critical thinking in STEM classrooms, yet reported gaps between beliefs and enactment due to time pressures, credentialing requirements, and disciplinary norms. Participants further described SEL and equity as deeply interconnected, framing SEL as a tool for amplifying student voice and affirming cultural identity. These findings highlight the promise of integrating equity-centered SEL within STEM teacher preparation to support early-career development.