A person who lives alone and avoids other people

One word substitute is Recluse.

Ascetic: characterized by severe self-discipline and abstention from all forms of indulgence, typically for religious reasons.
Recluse: a person who lives a solitary life and tends to avoid other people.
Unsocial: causing annoyance and disapproval in others; antisocial.
Agnostic: a person who believes that nothing is known or can be known of the existence or nature of God.