Ghoul Guy

A Ghoul Guy is a gray Boo Guy that lives in Shroob Castle in the game Mario & Luigi: Partners in Time. They attack in the same manner as their Boo Guy counterparts, albeit quicker and stronger. In battle, a Ghoul Guy rolls into Mario or Luigi, inflicting damage. Before a Ghoul Guy attacks, it splits into two transparent copies: one real, one fake. As the Boo Guy, only the real Ghoul Guy can attack. The Ghoul Guy points to the Mario brother it will attack.

Additionally, Ghoul Guys have an ability that Boo Guys do not possess; a Ghoul Guy can merge its spirit with another enemy. Although the Ghoul Guy gets sacrificed, the enemy affected has all of its stats increased massively, making it a far bigger threat. This is similar to Peps in Bowser's Inside Story.