It was Petta who resulted in Zetta getting his body back.
Broad Strokes of the game; most of the plot happened; but Pram and Trenia don't give him his body back until he encounters Petta. This is why he's in Book form in his Continuity Cameo and DLC until Disgaea 4: A Promise Unforgotten; which is the first Nippon Ichi game set after the PSP Updated Re Release. King Drake III lost his title (and most of his intelligence) to Pram in a freak transmigration accident.
Pram, being two years old at the time, didn't give a darn about it, ruling in his stead. She was already something of a prodigy beforehand, being able to formulate thoughts and communicate on a shockingly comprehensible level.