Elfleda Mirjasdottír (エルフレダ・ミルヤスドッティル), better known by her nickname Effie (エフィ), is the head maid of Rotte's manor and her personal wet nurse since Rotte was a baby. Her original nickname is Elfie (エルフィ), but it eventually becomes Effie in Rotte's house because a younger Rotte was unable to pronounce her name and would call her "Effie".
Effie is a Kýlbringa, a female only race that produces Guð Mjólk (グズミョルク, God Milk), a type of milk with medicinal properties that can be consumed by all races.
Effie has pink hair and green eyes. She normally wears a blue maid uniform with white stockings. She has long pointy ears somehow similar to a cow's and also posses a tail like a cow. While her big breasts are a characteristic of her race, according to Irmi, the continuous growing of her breasts is just fat at this point.
When Effie was 12 years old, her mother Mirja brought her along when Queen Mercelída requested Mirja's help to wet nurse a baby Rotte since Rotte was fussy with her mother's and ofther wet nurses' milk. Originally coming before producing Guð Mjólk to learn as an apprentice wet nurse, she starts producing Guð Mjólk as soon as she picks up Rotte, becoming since then her wet nurse.
As the years passed, Effie also became the head maid of Rotte's manor once Rotte was unable to deal with men and had to live separately from her mother.
It is revealed later in the manga that she has a serious interest in Naoya as a partner and procreator of her children. Since Kýlbringa is a female only race, they procreate with other races, normally picking their mates that they meet when working as wet nurses. To them, a mate is not a father and they aren't bound by family ties, so they are able to sire their children even with men that are married. Effie, however, tries not to think too much about it since she know that Rotte and Naoya aren't married yet, but she plans to take action once they are married and have children.
Effie has a caring personality like all Kýlbringa and fulfill her maid duties diligently. She deeply cares for Rotte. She is on occasions worried about her weight. Effie does not know how to swim.
Effie seems to easily get embarrassed, much more than her mother and sisters, but she has no qualms in getting in provocative situations when needed, such as when she had to breastfeed and masturbate Naoya when he was unconscious and severely inflicted by the Incubus' Dream Bondage.
While she does not show it, she is in fact enamored of Naoya, and it's only waiting for the formalization of Rotte's relationship with him to properly express it.