From the Super Mario Wiki, the Mario encyclopedia
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Each time Wario damages Cractus, it will stand immobile on the stage for a few seconds. While it does so, it drools a clear, toxic liquid which will transform Wario into Zombie Wario, which would waste valuable time. As soon as he transforms, Hotaru will be seen, slowly moving across the screen. Since light can reverse the effects of Zombie Wario, the player would need to try and touch Hotaru by jumping. As soon as the status effect is reversed, Hotaru will disappear. It will return again whenever Wario transforms into his zombie form.
While considered an enemy and indeed attackable, Hotaru has no negative effects on Wario and acts purely helpful to the player.
In a prototype version of Wario Land 4, Hotaru was always visible; however, the firefly did not emit an aura of light when Wario was not transformed into a zombie.