This paper presents a novel coupled-serial-chain (CSC) mechanism based multifunctional rehabilitation robot to assist the patients with lower-limb disability in their sit-to-stand (STS) movement. The CSC mechanism is constructed by coupling the joint rotations of a multilink serial chain with gears and operates by following the natural movement of human joints and limbs during the STS motion. The goal is to design an ergonomic and lightweight assistive robot with CSC mechanism that helps minimize the patient's joint/muscle strength and requires the least number of external actuators. A user-driven design framework is proposed to synthesize the CSC mechanism with type and dimensions personalized to the hip trajectory of an individual user. An example CSC mechanism is generated and verified by virtual prototyping and simulation. A physical prototype of the mechanism is built, which will be tested on the human subject to evaluate its maneuverability and physical compatibility with the subject's STS movement.