Mahou Shoujo Elena

With the help from a mysterious creature, Elena turns herself into a magical girl in order to beat Zoid, a tentacle monster, and save her sister, Emile. Since then, to protect her sister and to keep peace on Earth, Elena fights against ugly creatures as a magical girl; however, what awaits her is a harsh destiny: immoral days of naughty humiliations, and her battle eventually involves her sister and her dead mother. Can Elena exterminate the tentacle monster, or will she be driven crazy, losing all sense of reasoning to the ecstasy? (Source: DLsite, edited)