This paper presents a soft robotic ankle-foot orthosis (SR-AFO) exosuit designed to provide support to the human ankle in the frontal plane without restricting natural motion in the sagittal plane. The SR-AFO exosuit incorporates inflatable fabric-based actuators with a hollow cylinder design which requires less volume than the commonly used solid cylinder design for the same deflection. The actuators were modeled and characterized using finite element analysis techniques and experimentally validated. The SR-AFO exosuit was evaluated on healthy participants in both a sitting position using a wearable ankle robot and a standing position using a dual-axis robotic platform to characterize the effect of the exosuit on the change of 2D ankle stiffness in the sagittal and frontal planes. For both sitting and standing test protocols, a trend of increasing ankle stiffness in the frontal plane was observed up to 50 kPa while stiffness in the sagittal plane remained relatively constant over pressure levels. During quiet standing, the exosuit could effectively change eversion stiffness at the ankle joint from about 20 to 70 Nm/rad at relatively low- pressure levels (< 30 kPa). Eversion stiffness was 84.9 Nm/rad at 50 kPa, an increase of 387.5% from the original free foot stiffness.