When God created the heavens and earth and all that lives therein, he made a freewill choice to have that creation reflect himself, and in particular for man to resemble himself. To that end, he gave mankind a physical body made of the stuff that everything else was made of, including everything else living, and infused it with a spiritual animus that made mankind uniquely, specially in his image. Whatever a soul is (and I'm certain we really don't know what that is), it is something that came into being when the breath (spirit) of God was infused into the corpus of man.
It is that ethereal thing, the soul, which expresses itself through a physical being made for it, which makes a healthy human being a reflector of God's freewill and an expresser of it in its own accord. While an individual is in a body, that individual is beholden to that body for its expression of its soulish being. If a brain is damaged, malformed, underdeveloped, diseased or afflicted, the soulish power of freewill will be affected in its expression. God has created man as a discrete singularity made of body, soul and spirit, so that as the body goes, so goes the expression of soulish personhood.
A soul without a properly functioning brain will not express in the physical world the freewill it otherwise has the innate ability to. Without a body a soul is not a complete human, which is reflected, I think, in the crying out under the altar of those martyred souls in Revelation (and the fact that we get new bodies for eternity). When a person is intact and healthy, the existence of his or her soul, is what gives that person the capacity to express freewill. There are limits, of course, the most obvious being the physical laws of the universe, and what is more important in my opinion, the law of Spirit.
The Law of the Spirit determines the ability of the soul to express itself in harmony with God. After the Fall of Adam and Eve, spiritual death, or separation between man and the Spirit of God, was imposed upon mankind. Whatever sort of resting place the soul had been made to be for the breath of God, its connection to the breath of God was broken at that time and so freewill in mankind was incapable of willing in harmony with God. After Jesus rose from the dead and made the Spirit available to those who follow him, the born again have by that rebirth a renewed ability to will freely in harmony with God (though the not perfectly so long as they are in dying bodies).
So what is freewill? It is the volitional power human beings possess, which, among other things, allows them to be the expression of the image of God who possesses freewill in its ultimate sense. Though a soulish quality, it is communicated in the physical world through the auspices of the physical being (particularly, the brain). Natural human beings have no ability to express that capacity in line with God, supernatural human beings (the regenerated) have some capacity to, eternal human beings will be able to perfectly.