The default Character class does not work that well with this kind of setup. (It also won’t work well once you want to do things like make stompy feet actually stomp things when walking.)
Thus, the best option for this kind of movement is to define your ABigStompyRobot actor class as derived from APawn, and then add the appropriate visual components (some armature of Static Mesh components, Physics bodies, and/or Skeletal Mesh component/s.)
You can then implement the control, movement, and rendering code for this ABigStompyRobot pawn any way you want, and update it in, presumably, Tick(). The legs would be fixed in Actor reference frame, probably, so “turning with keyboard” would cause physics to turn the actor, which would turn the legs. If you’re really detailed, you simulate the actual walking using bodies. That’s pretty hard, but also looks much more realistic than the weightless foot-sliding of the current Mechwarrior series.
Get the controller, get its control rotation, translate that to a relative rotation compared to actor root, and set you “waist” rotation to that. You might want to clamp it to some minimum/maximum delta. Might even want to feed back to the controller to clamp its control rotation.