- particles with real-mass have time-like kinematics ($ds^2 > 0$).
- particles with zero-mass have light-like kinematics ($ds^2 = 0$).
- particles with imaginary-mass have space-like kinematics ($ds^2 < 0$) (tachyons).
So the question is pretty simple:
What would be the kinematics of a particle with both non-zero real and imaginary parts?