Devices, systems and methods supplement, repair, or replace a native heart valve. The devices, systems, and methods employ an implant that, in use, extends adjacent a valve annulus. The implant includes a mobile neoleaflet element that occupies the space of at least a portion of one native valve leaflet. The implant mimics the one-way valve function of a native leaflet, to resist or prevent retrograde flow. The implant restores normal coaptation of the leaflets to resist retrograde flow, thereby resisting eversion and / or prolapse, which, in turn, reduces regurgitation.