Astarte from Deception: Invitation to Darkness is actually Millenia from Deception II: Kagero.
This is actually very heavily implied. In Ending #1 of the latter game, Millenia ends up being turned into a blue-skinned immortal servant of a demon, and Astarte is a blue-skinned immortal servant of a demon... and this ending alone ends with "To Be Continued", but isn't actually followed up on in Kagero II
— although Millenia is a Bonus Boss
, she's in her human form. On top of that, naming Millenia "Astarte" will net you bonus Ark.