Mahou Shoujo Elena

With the help from a mysterious creature, Elena turned herself into a magical girl to beat Zoid, a tentacle monster to save her sister, Emile. Since that day, to protect her sister and to keep peace on the earth, Elena started to fight against the ugly creatures as a magical girl! However, what awaited 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 is she driven crazy and lost her reason by the ecstasy? (Source: DLsite)