The designated caretaker for the child of another person, Wet Nurses care for and raise those they have been tasked with caring for.
In the old days, wet nurses are hired to take care of a woman's baby. A wet nurse is a type of nursemaid. The name wet nurse came from the fact that they breast-feed the infant that they take care of. There are a lot of reasons why women of higher class hire wet nurses. Also, they believed lactation can suppress ovulation especially when the upper class families want to produce more offspring. Another reason is that the wife and husband have too many children to take care of. A wet nurse would provide help.
In anime, wet nurses are generally tasked with properly caring and looking after the child that they are currently tasked with caring for. In the Beelzebub franchise, wet nurses from the demon world are assigned by the Great Demon King to take care of and to help look after his sons, who will one day rise to power and inherit his throne. In Beelzebub, Hildagarde is assigned to Kaiser De Emperana Beelzebub, while his older brother Lord En is assigned Yolda, Izabella, and Satura.
Although rare in number, there are some examples of wet nurses in various hentai franchises and series. The most notable use of a wet nurse in a hentai-related franchise is in the Milk Money movie, in which the main character Kyoko takes a job as a wet nurse for other women's children after losing her unborn child in a horrible car accident.