Shoujo Tenshi Milcute

From Nagareboshi:
Miu’s father left her five years ago with a box. The box was locked, and Miu’s father took the key with him. on her birthday, he sent it back, and inside the box was a mahou shoujo transformation device! Now it’s Miu’s job to transform into Shoujo Tenshi Milcute and save the world!
From Nagareboshi:
This manga is about the forbidden love between Angel and Devil. (tenshi to akuma) There has been a war raging between Heaven and Hell for centuries–and it’s Miu’s job to find the “Heart of God” and stop the fighting.
EDIT: The author is putting this series on hold because she wanted to start Modotte! Mamotte Lollipop.