This study presented a mechanism overview of a novel modular knee exoskeleton, ACE-Knee, and the analysis of the design requirements by observing human knee-motion characteristics. The ACE-Knee exoskeleton consists of 1) base frame at waist, 2) a 3-DOF (degrees of freedom) passive spherical hip, and 3) a knee driving mechanism. The passive hip is designed based on a 3R spherical serial chain such that it has RCM (remote center of motion) capability. For designing a compact and efficient knee driving mechanism, it is realized by two crank-slider linkages where two sliders are coupled with a linear spring. The proposed kinematic structure enables the driving concept of the passive support by the linear spring and the active following by an actuator. In order to setup design requirements, gait experiments were performed for level walking and ascending/descending stairs. From the analysis of experimental results, unique motion and quasi-stiffness characteristics of human knee were identified.