@Thundercrash One idea would be a limited ability to redistribute body mass and augment various physical properties - rather than fully shapeshifting, it could go from thin and lithe with lean muscles (like a gymnast) to relatively short and stocky with a wrestling build (durable and heavy-hitting).
Throw in a short-range telekinetic field to supplement their physical attributes in different ways (meshing its psychic powers with its transformations) and you could potentially make it work - you'd basically have an omniproficient athlete that alters their specialty as needed :)
For balance reasons, you'd probably make it require a certain amount of time and concentration to alter their physique and field - enough that they could knock someone back and then switch while their opponent recovers, but slow enough to require an opening.
The simplest ideas for field options would be a telekinetic augmentation of any physical actions for Normal (run slightly faster, hit slightly harder, etc), weight reduction for Speed (run faster and jump higher), extreme telekinetic force applied behind punches/kicks in Attack (hitting like pistons) and extreme deceleration of anything traveling into the field for Defence.
You can tell that I enjoy reading stuff like Worm :P