The modulation of joint stiffness is a crucial mechanism that allows humans, and other vertebrates, to perform stable and efficient locomotion. Compliant actuation is an emerging branch of robotics, which aims to mimic the biological elastic properties of muscle fibers and series-elastic tendon structures. This paper presents the mechanical design and real prototype of a compliant 2-Degrees of Freedom (DoF) ankle-foot system for application in a humanoid biped robot. The proposed design centralizes the two DoFs in one single joint, while allowing independent control of the equilibrium position and stiffness in both DoFs. The presented prototype achieves higher ranges of motion in both sagittal and frontal plane than those expected in normal walking conditions, while at the same time increases the versatility of the ankle due to its compliant properties.