Cythina was the incarnation of the devil on the Rugrats if you think about it.

Angelica would always tell the children to never touch her and if she did, then she would hurt them. Angelica would talk to Cynthia because she's been possessed and wanted to find out what her lord and master wants her to do. And remember in the Rugrats movie when Angelica lost Cynthia? Angelica did everything in her power to recover Cynthia because she couldn't live without it.

Mind = Blown.