Who is Screwtape from The Screwtape Letters and what is their importance?

Screwtape is a senior devil who is the head of a department in Hell. He is a highly experienced, very subtle, very smart tempter who understands human weakness very well. While he can be affable and indulgent, understanding and even generous, his mind is never off the business at hand. He feels superior to Wormwood, his young nephew. When things are not going well and Wormwood tries to betray him, Screwtape shows his real nature: hateful, vindictive, without affection or mercy.