From the Super Mario Wiki
A Soul Bubble is a stronger relative of the Love Bubble encountered in Shroob Castle during the events of Mario & Luigi: Partners in Time. A Soul Bubble is similar to a Love Bubble, except its body is cream-colored, and its X-ed out eyes are green.
Soul Bubbles have the same attacks as Love Bubbles, except they attack much faster. Also, while the Love Bubble always circles the brothers a set number of times during its homing attack, the Soul Bubble's amount before attacking varies. Soul Bubbles can still heal other enemies in battle, just as the Love Bubbles. They can, however, revive enemies that were previously defeated much like Elite Troopeas from Mario & Luigi: Superstar Saga.
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