You can only get one thing from Anna’s shop: Anna’s Mementos. These are small bits and pieces of a larger illustration that can be viewed from the Extras option in the main menu. Collecting all bits that will complete an illustration will also unlock new scenarios in the History Mode section of the game. There are five illustrations in total, each requires 20 pieces to complete the full illustration. Upon collecting all pieces of an illustration, a very high level and difficult mission will appear in that illustration’s scenario. Completing any of these end-level scenarios is enough to unlock Anna as a playable character. Collecting mementos also opens up new Blessings in your Camp’s Temple option after collecting a certain number of them (5, 10, 25, and 50)
Upon meeting Anna’s conditions in a battle (for example, defeating 1,000 enemies), she’ll appear at a fixed location in the Map for a limited amount of time. That said, prioritize getting to her as soon as she appears; otherwise, you’ll have to meet her condition again in the same map on another battle in case you missed her. There’s one Memento you can acquire from most maps in story mode, while additional Mementos can be acquired by replaying Story Mode maps in Lunatic difficulty, as well as some specific mission maps in History Mode.