Tinykin

Each room of the house (Sanctar, Foliana, Balnea, Ambrose, and Celerion) has four letters that you need to track down, throw a purple tinykin at to pick up, and then deliver to that room's mailbox. The mailbox is typically found on the floor level, in the "town" areas of each room. The letters are found tucked away in corners. If you're exploring for all collectibles, especially the pollen, you will assuredly find all four letters in each room with ease. 

Explaining their locations in text would be really difficult, so refer to THIS PLAYLIST, which contains a video of each area, and in the Youtube description are timestamps for everything, including all letters. Full credit to WheatYT for the great videos.