This paper presents a paradigm for virtual prototyping of a class of one-of-a-kind assistive devices that can be customized to the human user. This class consists of passive, articulated mechanical aids for manipulation that are physically coupled to the user. We address the mechanism synthesis, design and optimization and its evaluation in a virtual prototyping environment that consists of a CAD model of the product and a customized model of the human user. In addition, we develop the theory and methodology for designing planar serial chain mechanisms with revolute joints coupled by cable-pulley transmissions. As an illustrative example, we consider the design and prototyping of a customized feeding aid for quadriplegics.