This study presents a systematic review of Embodied Conversational Agents (ECAs) in digital health, focusing on their design features and impacts on user experience. Following PRISMA guidelines, 33 studies were selected from Web of Science and ACM Digital Library. A dual-dimensional framework was developed: (1) design features—including appearance, communication style, empathy, gender representation, and personality traits; and (2) user experience dimensions such as usability, emotional engagement, trust, and cognitive support. ECAs are commonly used in mental health and chronic disease management. Key findings show that human-like cartoon agents with multimodal outputs (e.g., voice and gestures) and professional appearances improve trust and satisfaction. Empathetic design is especially valuable in mental health, while expertise cues are more effective in chronic care settings. This review provides evidence-based insights and design recommendations to support future development of ECAs tailored to various health scenarios and user needs.