(EMPTY) is an enemy in Mother 3. It does nothing at all. The enemy was possibly created due to garbage data or for testing purposes. In the Mother 3 Sprite Editor, the sprite used for NPC tests is called (EMPTY).
In the battle with this enemy, all that appears the Masked Man's battle background and a black square in the center, and his battle theme is Unfounded Revenge. (EMPTY) gives 0 EXP, making it seem like garbage data.