Algebra is a gatekeeper. For the 6% of students with dyscalculia (i.e., mathematical learning disabilities), an inability to pass algebra may significantly limit academic and career opportunities. Unfortunately, prior research on dyscalculia has focused almost exclusively on elementary-aged students' deficits in speed and accuracy in arithmetic calculation. This case study expands our understanding of dyscalculia by documenting how one college student with dyscalculia understood algebra during a one-on-one design experiment. A detailed case study of 19 video recorded sessions revealed that she relied upon unconventional understandings of algebraic quantities and notation, which led to persistent difficulties. The design experiment involved designing alternative tools to enable the student to reason about algebra, but the unconventional understandings persisted. This exploratory case study provides new insights into the character of difficulties that arose and persisted for one student with dyscalculia in the context of algebra and suggests the utility of documenting the persistent understandings that students with dyscalculia rely upon, particularly in understudied mathematical domains, like algebra.