Planet Guardian

Koyuki is a middle high schooler, and a cowardly one at that. She doesn't like it but she is the chosen one: she has to fight against evil aliens and send them back to their galactical prison. Luckily, she's helped by her brother who is a magical girl otaku and by Pyrosuke, her guide. This is a funny manga, unlike other magical girl ones which is adapted from a RPG-like video game.