Exoskeletons and wearable robotic systems have advanced substantially over the last decade for gait assistance, rehabilitation and load-carrying purposes. Currently, there are commercially available devices with stiff actuators. However, these actuators cannot adapt to their unpredictable environments. Thus, compliant actuators like series elastic and variable stiffness actuators have been implemented in exoskeletons and active orthoses. This paper presents a novel design and experimental characterization of a compliant actuator with adjustable stiffness for a lower limb wearable ankle robot (VS-AnkleExo). The proposed actuator is designed to mimic the behavior of biological ankle and maximizes the compliance between user and robot during a gait cycle. The adjustable stiffness of actuator is achieved through a controllable transmission ratio mechanism. Both transparency and tracking performance experiments are performed to demonstrate reduced the user-robot interaction force and improved the tracking performance of the proposed actuator, respectively. Experimental results showed that interaction forces between the user and robot are minimized in the transparency experiments, while the actuators proposed are able to track the given torque signals at various frequencies in the tracking experiments.