Ayano's Love Letter is a obtainable pre-hardmode material available to all difficulties.
It is obtained similarly to the Stylish Scissors or the Exotic Scimitar, requiring a NPC/NPCs to be killed. It is obtained by killing the Dryad, Nurse, or the Mechanic.
Out of being a material for the Bleeding Heart of a Yandere, it can also be used to "share your heart" to the Yandere . Instructions to sharing your heart are told on the Wedding Ring page.