Kurohime

Kurohime {魔砲使い黒姫, Mahoutsukai Kurohime?, lit. Magic User Kurohime) is a manga series by Masanori Ookamigumi.

It is about Kurohime, the infamous gunslinger buxom witch who challenged the Gods. She was defeated, and split into two parts - the hateful Kurohime and the compassionate Shirohime. Kurohime was reverted into a child, Himeko, and lost her ability to love. Because of that, Kurohime can only restore her former glory by loving someone - and that someone would be Zero, a boy she saved 10 years ago.