The Real Bat is Lucas' ultimate weapon in Mother 3.
It is thought to belong to Ness so serves as the second and last of Mementos Ness leaves in the world of Mother 3 along with the Friend's Yoyo, With Ness by your side defeat the evil Porky once and for all!
The Real Bat is found in a treasure chest deep below New Pork City. It serves as Lucas' strongest weapon, with an attack power of 100. As this is near the end of the game, and there is also no way out of Deep New Pork City, it's use is limited.