In this manuscript we examine how two students develop and express agency in and through high school physics. We tell the stories of two youth from a low-income, urban community to elucidate the important components of critical science agency in a physics context, and to situate a set of claims about how youth develop and express this concept. This research is part of a larger multiyear study of democratic practice in middle- and high-school science. We present three claims: (a) that critical science agency is intimately related to the leveraging and development of identity, (b) that critical science agency involves the strategic deployment of resources, and (c) that developing critical science agency is an iterative and generative process. Two university researchers have co-written this paper with the two students whose experiences serve as the cases under investigation, to provide both an "emic" perspective and student-focused voices that complement and challenge the researchers' voices.