The Fairly OddParents (2001)

S04E02 Vicky Loses Her Icky

Ever curious about the source of Vicky's ruthlessness? The answer is simpler than expected: it's all due to a pesky bug. When Timmy wishes for his cruel babysitter to be kind, a magical bug, embodying Vicky's wickedness, wriggles out from her rear, transforming her into a gentle, sweet-natured girl. Vicky is now an absolute angel. However, the sinister essence has to find a new host, leading the bug on a quest for its next victim.