Fast fashion in Indonesia faces increasing scrutiny over sustainability and ethics, yet purchasing remains driven by emotional and social influences. Understanding how brand attitude, social cause affinity, message strength, and corporate social advocacy perception influence purchase intention is crucial for aligning marketing strategies with evolving consumer values. This study investigates the direct and indirect effects of affective response and altruistic attribution in these relationships, identifying whether mediation is complete or partial within purpose-led marketing contexts. A quantitative survey of 1600 Indonesian fast fashion consumers from 30 cities, selected through purposive sampling, was analyzed using PLS-SEM to test the proposed mediation model. Findings reveal that brand attitude and message strength influence purchase intention only through complete mediation by affective response and altruistic attribution. At the same time, social cause affinity and corporate social advocacy perception show both direct and mediated effects. Theoretically, this clarifies the emotional and ethical mechanisms in purpose-led marketing. Practically, it guides marketers and policymakers to integrate emotional engagement with visible social responsibility, fostering ethical consumption and brand loyalty in Indonesia’s fast fashion sector.