In the seder (order) in which a Nefesh, Ruach, and Neshama enter into a person, in the first new gilgul, we will explain (with Hashem's help). Here, at the time the body of the person was born, and comes out into the air of this world, his Nefesh enters into him. If he makes his actions kosher, he will merit that a Ruach will enter into him (around the age of 13), and thus he is called a complete man, as we know.
And if he continues to keep his actions kosher moving forward, a Neshama will enter him at the age of 20, as is explained in בסבא דמשפטים. However, if he does not rectify completely his Ruach, a Neshama will not enter into him, and he will only have a Nefesh and a Ruach. Similarly, if he does not rectify his Nefesh completely, he won't have anything except for his Nefesh, and the Ruach and Neshama will remain above.