• Egoist is someone who is self-centered and thinks only of themselves.
• Extravert is someone who is outgoing and enjoys being around other people.
• Introvert is someone who is shy and prefers to spend time alone.
• Monk is a man who has dedicated his life to religious study and contemplation.

Introvert is the most appropriate word for this group of words because it specifically refers to someone who is shy and prefers to spend time alone. The other words do not have the same connotation of social withdrawal.