Source Text
And with this, I can explain to you a big reason for what we find in our midsts, that most of b'nei Adam have only merited a Nefesh, and only a minority of them in our later generations have merited a Ruach or a Neshama. And it is known, that Moshiach ben Dovid will not come until the Ruchos and Neshamot are rectified. But as we described, even the Ruach or the Neshama must be reincarnated into other bodies -- they each require the Nefesh of a Ger, and that is how even they were [already] rectified.
However, [returning to the person who already received his Nefesh, Ruach, and Neshama, but then he damaged them -- when he comes into a second gilgul with only his Nefesh], when his Nefesh merits to be rectified, [since he cannot receive his own blemished Ruach], he is able to receive another "Ruach" of a Tzaddik that is similar to him in his actions, and he can be a vehicle for the Tzaddik's Ruach.
Through this, he rectifies [the Tzaddik's Ruach], all the way until he is able to receive also a "Neshama" of a Tzaddik. Then, when this Nefesh leaves this world, if his own Ruach is not yet finished being rectified [e.g., it is still alive in this world in another body], between this time (until it is finished being rectified), the Nefesh goes with the Ruach of this Tzaddik to Olam Haba, and through the Tzaddik, it receives the shefa on the level of that Tzaddik.
When his Ruach is finished being rectified in the body of another person as we explained above, the Nefesh says, "I will go and return to my first husband," and the Nefesh reconnects with its Ruach (even if the Ruach is still in a body that is alive resting on another Nefesh, it will still return and join with it in the form of an ibur). So too with the Neshama once it is rectified, both his Nefesh and his Ruach will return to the Neshama (even in the form of an ibur if the Neshama is in another person), according to all that we have already learned.