In addressing age-related muscle strength decline among elderly individuals, the development of assistive devices for daily living becomes imperative. This paper introduces a passive knee exoskeleton designed to aid older adults during sit-to-stand movements. Inspired by knee joint biomechanics encompassing rotational and translational motions, this exoskeleton employs a cam mechanism. This mechanism facilitates thigh rotation within a linear slot, adjusting the exoskeleton's rotational center and effectively minimizing misalignment with the body's knee joint. As the upper part attached to thigh rotates within the cam mechanism, a spring in the lower part attached to the shank compresses via a crank, storing energy during the sitting phase and releasing it during standing. The evaluation involves using an instrumented chair and footplate to collect force data during sit-to-stand motions. Force sensor resistors are positioned to measure force and pressure distribution across various regions. Notably, the developed knee exoskeleton (KnExo) provides 25% and 31% assistance of body weight under no-loaded and loaded (with a 20kg load) conditions, respectively, according to initial functional evaluation. Furthermore, compared to a revolute joint, the proposed knee exoskeleton exhibits a substantial 51% reduction in misalignment with the user's knee joint. This underscores its potential as an effective assistive device for elderly individuals coping with diminished muscle strength.