The empirical character — the totality of a person's dispositions, capacities, and tendencies — is the will as it has particularised itself in this individual. It is not acquired by experience or shaped by upbringing in any fundamental sense; it is the person's given nature, present from the beginning and merely revealed by the circumstances life provides. Education can teach someone to mask or redirect their character; it cannot alter it. The selfish person who appears generous is exercising selfishness in a more sophisticated form. The apparently brave person who proves cowardly under extreme pressure was never truly brave.
This has a disturbing but consistent implication: virtue cannot be taught. You cannot make a person compassionate by lecturing them on duty or training them in moral habits. Either the compassion is already there — part of their inborn character — or it is absent, in which case no amount of moral instruction will produce it. What education and culture produce is the appearance of virtue: behaviour that conforms to social norms regardless of inner motivation. True moral worth, for Schopenhauer, is the expression of a will that is by nature more transparent to the suffering of others — not the product of effort or decision.
The doctrine of the immutability of character is developed in §55 of The World as Will and Representation and in the ethics essays in The Two Fundamental Problems of Ethics. It draws on Schopenhauer's account of the will as thing-in-itself and its relation to the individual empirical character.