This paper presents a novel index finger exoskeleton mechanism for patients who suffer from brachial plexus injuries, which takes advantage of our previously proposed rigid coupling hybrid mechanism (RCHM) concept used for robotic tail mechanisms. The core idea of this concept is to drive the (i+1)-th link using the motions of the i-th link, instead of the traditional way of transmitting motion directly from the base. This specific configuration allows designing a single degree of freedom (DOF) bending mechanism using a low-profile rack and pinion mechanism and makes the proposed exoskeleton system compact, lightweight, and portable, which are highly desired features for daily usages of exoskeleton gloves. The mechanism is optimized to mimic the grasping motions of human fingers and the sensitivity analysis of its critical design variables is then conducted to explore the performance of the optimization results. The results show that for the current design, the tip position accuracy is mainly affected by the distance between the rack and the corresponding joints. A proof-of-concept prototype was built to verify the novel mobility of the proposed mechanism and to evaluate its performance on a human finger. The index finger exoskeleton experiments demonstrate the new mechanism's ability to grasp small objects.