You are a Magical Girl who has overcome your despair. Instead of becoming a Witch when your Soul Gem fills up, you can summon a fraction of your Witch’s power and merge it with your Magical Girl form. This is called a Doppel and it is a hybrid form that combines both your Magical Girl and Witch forms. A Doppel is much more formidable than a Magical Girl alone, but it does not reach the full potential of a Witch. Your Doppel is closer to a second stage to your Magical Girl form with the theme of your Witch.
Using your Doppel gives you access to some of the abilities and traits of your Witch, such as unique powers based on your Witch. Your Doppel also reflects your personality and wish but in a twisted and distorted way. Your Doppel has a different appearance from your Witch, but they are still connected to each other.
Your Doppel form is powered by the curses that have accumulated within your Soul Gem, and automatically activates should you have enough to start to turn into a Witch if you hadn't had this ability. When your collected curses run dry, so does this form.
(This is based off of the game continuity, so overusing your Doppel will not result in it taking you over permanently.)