A withdrawn person is very quiet and shy, keeping their thoughts to themselves and not joining in. The picture inside the word is someone stepping back out of the room: they are still there, but they have pulled away from other people.

The important thing is the feeling behind it. 'Quiet' is neutral, 'shy' is common and harmless, but 'withdrawn' usually suggests something is wrong. It is the word teachers, doctors, and worried parents use: 'He's become very withdrawn since his father died.' 'The child was withdrawn and would not look at anyone.' It often points to sadness, depression, or shock. So it is not a nice word for someone's personality; it is closer to a warning.

Because of that, you will very often see it with 'become' or 'seem' — it describes a change, not just how someone always is: 'She has become withdrawn.' You can also use it before a noun ('a withdrawn teenager'), though after the verb is more common.

Be careful with the spelling, because 'withdrawn' is also the past participle of the verb 'withdraw' (to take back or take out), and that use is everywhere: 'money withdrawn from the account', 'the offer has been withdrawn', 'troops were withdrawn'. Same word, completely different job. The sense here is the personality one.