Because of Doremi’s constant mishaps in magic, Majo Rika refers to her as an “ojamajo” (troublesome witch).
On her way, Doremi is joined by her two best friends. Hazuki Fujiwara and Aiko Seno, as well as her younger sister, Pop. While taking tests to become better witches, at the same time, Doremi and her friends help out their classmates and friends with their own problems via magic, as well as battle Majo Rika’s rival, Majo Ruka.
All is going well until they meet Onpu Segawa, a child idol, who does not share the same morals in using magic as them. Eventually, Onpu allies with Doremi and the girls and even saves them from turning into witch frogs at the cost of her own consciousness.
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Source: http://magicaldoremi.wikia.com/wiki/Magical_DoReMi