The Ethical role of the Forms lies in the fact that the Forms found in the Realm of Being are the perfect ideals of that of which they are the Forms. For example, "the Just Itself" can in no way be unjust, it is purely, fully, really just; it is "perfect" justice. Thus the Form serves as a "yardstick" or "natural standard" against which to measure the degree of justice in this or that person's life. This standard has a objective reality quite apart from human opinions about justice and injustice; it determines what really is just, not merely what appears to be just in the false (and changeable) opinions of people. So all of the virtues, such as "piety" or "courage," etc. which Socrates was questioning his fellow Athenians about now emerge in Plato's theory as Forms. The definitions Socrates sought (but did not find in the eraly dialogues) required, in effect, a mental "vision" of the Forms. When we have ethical knowledge, knowledge of what is really and truly "virtuous" and what is not, it is because our mind "recognizes" the Form as a natural standard against which to measure the virtue of this or that person or deed in the Realm of Becoming as more or less virtuous. Thus the philosopher-king, who, by definition, is the person who has such perfect knowledge, will always know what is good, right, or just.
At the end of Meno we saw that Socrates argues that since by definition "virtue" (arete') is that which brings a person to the goal of living, "happiness" or "eudaemonia," it follows that virtue must be beneficial. In other words, arete' as that which brings well-being, is essentially good. But all of the separate so-called "virtues" (like pitey, courage, friendship, moderation, generosity, etc.) are themselves good; there must be, therefore, some common essence in which they all share which makes them all good. Whatever that is, by the language of Theory of Forms, it must be the Form of The Good. In other words, to know what the virtues are requires not merely a mind's seeing of the Forms of the particular "virtues" but an understanding of why they are virtuous, what they all share in common which makes them all good. And that, in Plato's language, requires the mind's grasping The Form of the Good. The mental grasp of that Form is the supreme goal of the lover of wisdom. It is the highest of the Forms, that which gives them all their "goodness," their value. Thus it can be regarded as the source of all value. The direct mental vision of The Form of the Good is what defines "the philosopher king"; it is knowledge of arete' itself.