Obtaining Kafei's Mask
Link simply has to walk up to Madame Aroma in the Mayor's Residence in East Clock Town. She thinks that Link is a professional detective that will help her find Kafei, her elusive son, and then hands Link Kafei's Mask so that he can ask residents around the town about the missing youth.
Kafei's Mask plays an integral role in the Anju and Kafei Side Quest, where the reward is the Couple's Mask. If Link goes into the Stock Pot Inn and speaks to Anju wearing the mask between the times 2:15 PM and 3:50 PM on the First Day, he will be handed the Room Key by mistake, instead of the Goron by the same name as Link. Then, wearing the mask, Link may speak with Anju where she tells him to meet her in the kitchen later that night.
Through the use of Kafei's Mask and initiating the Anju and Kafei Side Quest, there are a few other things to be acquired. Chateau Romani can be obtained after delivering the Letter to Mama to Madame Aroma on the Final Day as an optional part of the quest. Using Kafei's Mask to start the Anju and Kafei Side Quest will also result in gaining the Couple's Mask in the end.
Additionally, the Couple's Mask can then be used to stop the pandemonium in Mayor Dotour's office in the Mayor's Residence, resulting in Mayor Dotour giving Link a Piece of Heart.