In this work, we demonstrate a freeform metasurface with novel birefringent properties. Traditionally, birefringence is considered as an intrinsic material property and its behavior typically limited to what is known as linear birefringence. In this work, by using a photonic inverse design approach, we are able to realize arbitrary birefringence in which the eigen-polarization states can be linear, circular or elliptical. It also features convenient angle tunability - by changing the angle of incidence, the birefringent property can be continuously adjusted. The novel polarization behavior and multifunctionality open up new possibilities in polarization imaging, quantum optics and other areas.