Planet Guardian (2001)

In a starry night, an extraterrestrial entity named Pirosuke appeared in front of a fourth-grader Koyuki Kisaragi and gave her power to transform into a Planet Guardian and protect the Earth from evil aliens. Yet, not a single alien has ever appeared in five years. Now, a long-awaited evil alien has finally arrived, but Koyuki, now a ninth-grader, has no interest to fight for the sake of humanity. She just wants to be an ordinary girl.

Source: www.anisearch.com/manga/4823