In the Catholic Tradition does there exist a teaching on the purpose or role of the human soul in the makeup of a human being? I could also settle for a theologian's thoughts and writings on the subject as well.
The Catechism of the Catholic Church has this to say with regard to the soul itself:
In Sacred Scripture the term "soul" often refers to human life or the entire human person. But "soul" also refers to the innermost aspect of man, that which is of greatest value in him, that by which he is most especially in God's image: "soul" signifies the spiritual principle in man. [363, emphasis in original]
The Catechism emphasizes the unity of the spirit and the body, saying that man "is a being at once corporeal and spiritual" (362), and that "spirit and matter, in man, are not two natures united, but rather their union forms a single nature" (365). Perhaps most relevant to your question, the Catechism teaches that the human body "is a human body precisely because it is animated by a spiritual soul".
Although the presence of the spiritual soul cannot be observed experimentally, the conclusions of science regarding the human embryo give “a valuable indication for discerning by the use of reason a personal presence at the moment of the first appearance of a human life: how could a human individual not be a human person?”. Indeed, the reality of the human being for the entire span of life, both before and after birth, does not allow us to posit either a change in nature or a gradation in moral value, since it possesses full anthropological and ethical status. The human embryo has, therefore, from the very beginning, the dignity proper to a person. [emphasis added]
N.B. Some have taken the church's teaching of creationism of the soul (see Catechism 366) and argued that we don't know when God creates the soul in each person. That's likely the argument made by the abortionist you mention, but it's also the exact argument rebutted in Dignitas Personae. Human embryos, according to Roman Catholicism, are always human persons, and thus have souls from their very beginning.
Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.
"Form" is "the actualizing principle that makes a thing to be what it is." Thus, a body without the substantial form of a human being (a soul) is a corpse.
The Oxford English Dictionary defines form as:
In the Scholastic philosophy: The essential determinant principle of a thing; that which makes anything (matter) a determinate species or kind of being; the essential creative quality.
This use of form (Aristotle's μορϕή or εἶδος) and matter (ὕλη) is a metaphorical extension of their popular use. In ordinary speech, a portion of matter, stuff, or material, becomes a ‘thing’ by virtue of having a particular ‘form’ or shape; by altering the form, the matter remaining unchanged, we make a new ‘thing’. This language, primarily applied only to objects of sense, was in philosophical use extended to objects of thought: every ‘thing’ or entity was viewed as consisting of two elements, its form by virtue of which it was different from, and its matter which it had in common with, others.
Read St. Thomas's short work On the Principles of Nature (De principiis naturæ) for more information on matter and form (hylemorphism).