Mimina is Princess Helena's younger sister who decides to come to Earth posing as Kanou's little sister so she can chastise Ririka for not doing a good enough job.  Mimina develops a crush on Seiya, much to Ririka's annoyance.  Mimina is accompanied by Maruru, who serves as her adviser and servant.

The Evil Forces of Dark Joker are closing in on our planet after having destroyed the beautiful planet of Queen Earth. Now, 10 year old Moriya Ririka, with the help of her childhood friend Seiya and the mysterious Kanon, must transform into the Nurse Angel and find the elusive Flower of Life, the only way to defeat the evil forces. The Flower of Life, that once bloomed all over the Earth, is where no one thought it ever would be. And Ririka must make the hardest decision of her life in order to acquire it and rid the universe of evil once and for all.
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(Source: Anime News Network)

Ririka is an average student that can, in fact, transform into "Nurse Angel" by using a special nurse-like hat given to her by her senpai Kanou (who is, in fact, from another dimension/world connected to this one). Ririka's fellow classmate and neighbor is Seiya, who follows her and helps her as much as he can. Aside beating evil monsters.