Lower limb exoskeletons have been successfully used in robotic-assisted rehabilitation. However, the design limitations of exoskeletons mechanics, such as weight and the lack of kinematic compatibility relative to the user's joints, limit the outcomes of treatment. To address these shortcomings, this work presents the design of a magneto-rheological fluid-based actuator for a knee exoskeleton, namely MRKE. The system was designed to ensure better mobility of the user, presenting high backdrivability and kinematic compatibility with the knee joint. The power train of system is a BLDC 70 W motor integrated to a harmonic drive gearbox. To improve kinematic compatibility relative to the user's knee, a four-bar crossed linkage mechanism (FBLM) was designed to follow the trajectory of the knee center of motion. A customized MR clutch was projected to decouple the motor-reducer from the FBLM, thus enabling high backdrivability. Preliminary results showed a small error (< 3 mm) between the FBLM and the knee center of rotation. Moreover, the MR clutch allowed for low backdrive torque (1.0 N.m) compared to the torque to backdrive the motor-reducer (16.6 N.m).