*SPOILERS*
If you pick choice 3 in FMG003 where you basically turn down wanting to work out with Akira this will unlock the possibility of seeing the Natsuko route scenes later in the route (previously this choice would have just sent you into an endless loop), but this does not commit you to her route yet even if you do the scene. There is currently only one scene, but more are planned and there will eventually be a point where the routes split off permanently.