Menya Moonspell is a character that is unlocked by Evolving the Silver Wind weapon (Which requires the Pummarola at level 5 and Silver Wind at level 8 before it can be evolved using a chest).
You can also unlock her by entering the spell "YoungAtHeart" into the scrolls of morbane. Just a reminder that even after unlocking her you will need to buy the character, the cost may vary depending on how many characters you've unlocked in the past.