ReboStep is the world’s first apparel-style wearable robot that synchronizes with user movements to assist knee extension. It delivers 15 Nm of knee assistance, about 40% of the knee support used during walking, to reduce muscle fatigue, joint load, and pain. Existing personal wearable robots rely on bulky, exposed exoskeletons, creating a psychological burden that often leads target users to abandon them in daily life. To eliminate this visual stigma, ReboStep uses tendon-driven motors and anisotropic stiffness patterns on flexible substrates. By replacing rigid metals with highly flexible materials, it achieves an ultra-thin design with a 1.5 mm main body frame and a joint section measuring no more than 15 mm thick. Worn discreetly beneath everyday clothing, ReboStep minimizes technological visibility and social discomfort, helping older adults and people with mobility limitations move more freely and confidently during walking, stair climbing, sit-to-stand transitions, and hiking.