I think I read somewhere that the brain is the organ that consumes BY FAR the most energy, accounting for up to 20 % of total energy consumption in the body.
Also, on agency and freedom, Sartre tackles this when he says that we are what we make of what life gives us, and in his existential literature he delves into the tension between our facticity (that which we are born with, that we have no control over and acts as gravity pulling us down) and our instinct of freedom, our process of self-creation which we do have control over.
A Concrete Self - addresses some of these "ghost in the machine" issues, which really are inherited from a faulty, Platonized interpretation of reality. The essay reconciles a modern materialist theory of self with Epicurus' teachings in his Epistle to Herodotus according to which bodies gain complexity as they grow and gain particles, and with added complexity they generate "relational" or secondary properties beyond the conventional "atoms and void" properties--both of which are observable and real. The real, observable self emerges organically as symbiosis, as complex systems (like all else in nature), not as a Platonic "Casper" without a body.