The Shell Fruit is an item in Momodora I.
In Momodora I, the item is nothing but a collectable.
The item can be obtained in the last room of Area 4, before the room with the save point. It is hidden in what appears to be solid rock in the upper right corner of the screen. Just jump into the rock and you will enter a secret stash with the item in it.
In Momodora III a similar item, called the Slice of Shellfruit, exists. It can be used to upgrade your Maple Leaf, granting you stronger attacks.