This dissertation explores the potential of robot "skin'' as a dynamic, communicative medium for human-robot interaction (HRI). Moving beyond the conventional view of robotic skin as a passive, protective layer, we envision it as a functional, interactive, and dynamically changing component of the robot's system. Drawing inspiration from nature and living creatures, we design and prototype concepts that illuminate the potential of robotic skin within human-robot interaction contexts. We hope this exploration will expand the uncharted design scope of robotic skin, informing behavior design and inspiring further exploration into the realms of expressive, tactile, and transformative robotic skin. In particular, we highlight three aspects of robot skin in HRI: expression, sensing, and temporal change. The "Expression" chapter explores the potential of texture-changing skin as a design modality for expressing robotic emotions and characters. The "Sensing" chapter introduces ShadowSense, a vision-based tactile-sensing approach that augments the sensing capability of a robotic skin at low cost, using shadow image processing and classification. The chapter "Temporal Change" challenges the concept of never-aging robots by presenting plant-driven robots that embody growth, aging, and decay. This includes the skin that grows taller and develops dark spots over time. We hope the physical transformation and inevitable decay of social robots can elicit empathy and care, and incorporate a sense of transience in long-term human-robot relationships.