Marie is the added character in the Golden version of Persona 4. She acts gruff and brusque towards others, but sometimes seems quite naive and innocent.
Marie will help you, through the Velvet Room, by registering your Skill Cards and then selling them to you, Configuring your SOS messages and finally bringing her Social Link to the maximum will allow you to open an extra Dungeon.
How to unlock the Confidant
You can start Marie's Social Link by visiting the Velvet Room on April 18th and interacting with her. His last day to improve the Social Link is the 31 October. You will have to bring his Social Link to the maximum by February to unlock the additional dungeon.