Love Letter is an item that can be found in the first three Momodora games, oddly they have a different function in every one of them.
The Love Letter is a collectable item in Momodora I.
Like the other collectable items, the Love Letter itself does nothing. It's sole purpose is to be collected.
The Love Letter can be found in Area 7, when following the normal path to progress the game. It is in the third room of Area 7, near the exit of that room in the upper-right corner. You can already see it from afar, so its rather hard to miss.
Obtaining a Love Letter will permanently increase your maximum health by one heart.
Every main area in the game has at least one chest with a Love Letter inside
Even though they are optional it is recommended to collect a vast majority, if not all, of them.