The definition and meaning of the fruits of the spirit in the bible, can be seen in two ways: first as the ultimate goals of life, that is to say where we direct our life, where the intention of our actios goes, the fruits are: love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness, self control. The fruits of the spirit are the goals of the righteous life: "Do unto others as you would have them do unto you", Matthew 7:12.
Also the fruits are a description of the character of God. The character of God relates us to the concept of image. The Bible tells us: "Let us make man in our image, after our likeness ..." Genesis 1:26, as we noted Thomas Aquinas in the Summa of Theology the man is made in the "image",  after our "likeness" , that means that in man there is a certain likeness and certain image of God as the whitish to the white.
Image means perfect likeness, be full image of God means that they have all the traits of the character described by the fruits of the spirit, the holiness is the image of God. And what is the likeness? Is the conformity or agreement in the essence.
Consider this: there are two concepts in metaphysics: substance and essence. The substance is that of which it is predicated, examples: the substance of God is wisdom because talk about wisdom is talk about God, the substance of man is the person.
Essence is the main or distinctive feature of something, example the essence of God is Wisdom, the essence of man's is the reason, man is defined as rational animal. The espirtual life is born from the difference between reason and wisdom. If this difference did not exist the spirituality would be a trivial thing, it would be no need to be taught, as in the case of the Angels, as they are created by God in the state of grace.
Being likeness of  God means sharing his essence, and that is achieve surpassing the natural love of the soul that is proper to the reason, or to say to receive the spiritual Love, Agape, or Love for the good. The retitude is the way to the full likeness. The righteousness, or gift of righteousness, is defined as the Golden Rule: "Do unto others as you would have them do unto you."
The image is reached perfecting the similarity, through perseverance and the contemplative activity we perfect our likeness to God gradually getting the traits of the spirit.