Puffy is a robotic companion that has been designed in cooperation with a team of therapists and special educators as a learning & play companion for children with Neurodevelopmental Disorder (NDD). Puffy has a combination of features that support multisensory stimuli and multimodal interaction and make this robot unique with respect to existing robotic devices used for children with NDD. The egg-shaped body of Puffy is inflatable, soft, and mobile. Puffy can interpret child’s gestures and movements, facial expressions and emotions; it communicates with the child using voice, lights and projections embedded in its body, as well as movements in space. The paper discusses the principles and requirements underlying the design of Puffy. They take into account the characteristics of NDD and the special needs of children with disorders in the NDD spectrum, and provide guidelines for designers and developers who work in socially assistive robotics for this target group. We also compare Puffy against 21 existing commercial or research robots that have been used with NDD children, and briefly report a preliminary evaluation of our robot.