Physically, the have small horns, long pointy ears and also a tail, all similar to a cow's. They all have large breasts which develop early.
A special characteristic of the Kýlbringa is the production Guð Mjólk (グズミョルク, God Milk), a type of milk with medicinal properties that can be consumed by all races. A Kýlbringa can start lactating without stimulation since their late childhood. Due to this, they normally work as wet nurses.
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. The Kýlbringa are motherly beings that display natural affection for those appearing smaller or younger without consideration of race.
The Kýlbringa are a national treasure and receive full protection from the Kingdom of Ygvar. In the past, there was a very high infant mortality rate. The Guð Mjólk produced by Kýlbringa contains antibodies against diseases, is never repulsive does not cause diarrhea and remains delicious and nourishing for all ages, so it was the primary means of defense against infant mortality. Because of this high grade milk, the Kýlbringa are highly prized even in the present. The Guð Mjólk also has hints of different fruit flavors depending on the Kýlbringa that produced it.
The "Kýl" part of the name appears to be a mistake caused by the perception in Japanese of /r/ and /l/. The proper word seems to be "Kýr". If this is indeed correct, "Kýlbringa" should be "Kýrbringa" and it would be a combination of the Icelandic words "kýr" ("cow" in English) and "bringa" ("chest" in English), the "Cow chest".