Identity currently features as a prominent theme in digital multimodal composition (DMC), given the significant impact of DMC on identity expression. However, L2 learners' writer identity development and fluctuation in this type of textual composition needs further exploration in light of the absence of clearly established DMC-specific identity categories and a detailed account of writer identity expansion and restriction processes therein. This study attempts to fill this void by scrutinizing how three 11th graders learning English as a foreign language in Spain crafted their writer identities in English in a 16-week genre-based digital storytelling (DST) intervention that followed the genre-based systemic functional linguistics tradition of L2 writing, and included instruction on attitude (Martin & White, 2005). Qualitative data analysis of multi-source data followed Ivanic's (1998) writer identity framework, attitude categories, and key notions in multimodality, as well as the DST and identity literature. Results indicate students' awareness raising of such categories throughout digital story writing, and general preference for appreciation and judgment in writer identity construction, which was contingent upon certain factors. Students' writer identities and their fluctuation revealed types of specific DMC identity categories, and expansion and restriction processes, which illustrated their multimodal composing identity formation and transformation in digital story creation.