I don't know if Soul's suit is a full 3D model or a 2D rig with 3D hands, his face and hair looks like it's drawn.
It would make more sense because between Maka and Soul's normal outfit that's already two custom 3D models that need to be made, and possibly even three if Maka's final form is included in the fight (which seems likely). Soul and the piano don't need much animation so it makes sense to just make him 2D for those scenes.