Here we have Madeline and Nanny. Nanny is Madeline's summoned demon slave.

Madeline thought one she needed a demon maid. Only problem is equal and opposite exchange. Madeline wanted to be cleaned up after and taken care off. So she paid the price with her adult body. The more dependant Madeline becomes the more loving and loyal Nanny becomes.

That collar around nanny's neck is called the Ever Milk. In exchange for her walking Madeline gets an unlimited supply of milk.