Human support is thought to increase adherence to internet-based interventions for common mental health disorders, but can be costly and reduce treatment accessibility. Embodied virtual agents may be used to deliver automated support, but while many solutions have been shown to be feasible, there is still little controlled research that empirically validates their clinical effectiveness in this context. This study uses a controlled and randomized paradigm to investigate whether feedback from an embodied virtual agent can increase adherence. In a three-week ecological momentary assessment smartphone study, 68 participants were asked to report their mood three times a day. An embodied virtual agent could mirror participant-reported mood states when thanking them for their answers. A two-stage randomization into a text and personalized visual feedback group, versus a text-only control group, was applied to control for individual differences (study onset) and feedback history (after two weeks). Results indicate that while personalized visual feedback did not increase adherence, it did manage to keep adherence constant over a three-week period, whereas fluctuations in adherence could be observed in the text-only control group. Although this was a pilot study, and its results should be interpreted with some caution, this paper shows how virtual agent feedback may have a stabilizing effect on adherence, how controlled experiments on the relationship between virtual agent support and clinically relevant measures such as adherence can be conducted, and how results may be analyzed.