Astral Gremory is the neglected brother of Sirzech. As the years went on Astral grew more and more powerful. However this power still didn't get his family to notice him. So after all the credit for his accomplishments are given to Sirzech and that's when Astral jas had enough. He leaves the devil and doesn't return until years later.

I don't own dbz/s or Dxd