This longitudinal case study, supplemented by cross-sectional comparisons among five groups of writers with differing backgrounds, investigates how Natsu, a Japanese multilingual writer, developed her L1, L2 (English), and L3 (Chinese) writing competence over two and a half years. To create a comprehensive picture of this multilingual writer, the study examines three aspects of writing: written essays (linguistic development and text features), composing processes, and individual/social factors (attitude and identity). Multiple data sources, both elicited and naturally occurring, include argumentation essays written in the three languages, retrospective stimulated recall of pausing behavior, interviews, and natural observations. Qualitative and quantitative analyses of the data reveal the writing development of the multilingual writer in the three languages over time and influential factors affecting that development, including past experience and individual perceptions. The findings suggest that: (1) both commonalities and distinctions co-exist in the textual, process, and social aspects of her writing, (2) the writer's personal and cultural identity affect her text construction and composing process, and (3) boundaries become blurred among both the textual and the linguistic features in the three languages. The results imply that partially overlapping theories of multicompetence, genre, and identity can help elucidate the unique character of multilingual writers. (Contains 8 tables and 5 figures.)