Yes, I'd say that is his singular defining attribute above all other attributes. It is written, "God is love." If we read his commandments in the New Testament, they are all in the format of "Love ..." as in "Love God" and "Love thy neighbor" and "Love thy enemies" and "Love one another." If you weigh his actions, these also sum up to love, "For God so loved the world, that he gave his only begotten son..."
Holy just means set apart pertaining to God. Trying to use Holy to define God is a circular definition. Light is a metaphor, and besides this God existed before light, he created light and brought it into being. Does light really describe who a person is? Light is not a person, it is neither good nor evil.
Is it not written, that it is not what enters a man that defiles him, but what proceeds out of his mouth? What makes us clean or unclean is what is inside us in the spiritual sense. What defines a person is his character. And if we are on the subject of the character of our God, what truly defines him above all other traits, I would say the answer is love.
Anyone with the power to create worlds out of nothing can be a god and call himself God. But who is our God? Once you establish that we are a creation and there is a Creator, the next step is discovering who that God is.