If a person rectified his Nefesh, and came back to receive and complete his Ruach, but during that gilgul he sinned, then his Nefesh will not be affected in such a way that it would be forced to come back by itself to become rectified once again.
Once the Nefesh has been completely rectified and he has returned to work on Ruach, the Nefesh is protected against any further damage. Otherwise, the rectification process could conceivably go on forever.
Rather, because he now has a Ruach, the sin will only damage the Ruach, and only this will need rectification.
Therefore, if an additional reincarnation is necessary to rectify the Ruach, then both the [rectified) Nefesh and the [blemished] Ruach will come back again together. This will continue until the Ruach is rectified, after which he will have to die in order for the rectified Nefesh and Ruach to reincarnate with the Neshama. If he has accomplished this and then sins, then it will only damage the Neshama, just as we explained with respect to the tikun of Ruach.
However, there are different levels of rectification, and the above-mentioned procedure would be altered accordingly:
They will reincarnate together until the Ruach is rectified
It can also happen that the Nefesh becomes rectified and purified to such a great extent that it need not come back again with the Ruach for the rectification of the Ruach. Rather, the Nefesh remains Above in a place fitting for it, "bound up with the Bundle of Life."
One of the different levels of rectification is so complete that the level of soul that has been rectified does not need to ever come back again.
In such a case, the Ruach would have to come back alone to rectify itself. However, this is not possible.
The Ruach cannot come down into a body without the presence of a Nefesh. In this case, however, the Nefesh that has undergone such a complete tikun will not return to reincarnate.
Therefore, it reincarnates with the Nefesh of a convert, as it says in Sabba d'Mishpatim [in the Zohar]. They will reincarnate together until the Ruach is rectified.
This Ruach, therefore, will come down to join the Nefesh of a convert, and they will reincarnate together until the Ruach is completely rectified.
Once that is achieved, then the person dies and the first Nefesh comes back to join with it [the Ruach] in order to receive and rectify the Neshama.
Or, the Ruach may come back by itself with the Neshama until the Neshama is rectified, after which time the three of them no longer need to return and are instead "bound up with the Bundle of Life," as is fitting for them.
Once all three levels of soul Nefesh,Ruach, Neshama are rectified, there is no longer any need for gilgul
It was said previously that the Ruach does not enter a body without the presence of a Nefesh. In this case, however, where the Ruach was completed when it was together with the Nefesh of the convert, then the rectified Ruach may serve as the vehicle and base for the entry of its Neshama, as if it were both Nefesh and Ruach. In this case, therefore, the presence of a Nefesh is not absolutely necessary.
In any case, once all three levels of soul: Nefesh, Ruach, Neshama are rectified, there is no longer any need for gilgul, and all three may remain "bound up with the Bundle of Life," as is fitting for them.
The Arizal now returns to consider the fate of the Nefesh of the convert that has been the vehicle for the Ruach that came into the world without its Nefesh because the latter had undergone such a complete tikun.
The Nefesh of the convert that was joined to the Ruach has helped it to perform good deeds in this world, and has been a vehicle for it in this world; through their union the Ruach was able to achieve its tikun. This Nefesh of the convert will also become elevated with the original Nefesh of this particular Ruach. The two of them will be on the same level in the World to Come, like "neighbors," and they will never part from one another.
[Commentary by Shabtai Teicher.]