The Lineage 2 Character PIN is a secondary password to help keep your Lineage 2 game account more secure. It is designed to thwart keyloggers and increase the security of your account.
You will still need to use your game account password to log into the game. Then, after you've set up your Lineage 2 Character PIN from your character selection screen, this PIN will also need to be entered in order to log into the game with any of your characters. You will not have to enter it again after logging out to Character Selection (without quitting the game) and when trying to log in again as the same character or another character. If you quit to desktop, you will have to re-enter the PIN.
All characters on an account, even across different servers, share the same PIN.