She is the half-sister of Belldandy and Skuld . Urd is the daughter of Belldandy and Skuld's father (a Heaven-born angel) and Lind (The Devil), making her half-angel and half-demon, and the Biblical Antichrist.
Despite this, she openly embraces her angel half and rejects her demon half, seeing herself (and being seen by others) as an angel.
She first came to Earth (specifically, to Nekomi City in Japan) to trick Keiichi into kissing Belldandy. Belldandy found out about the trickery and was rightfully angry, causing the two sisters to start fighting. The fighting was stopped by the Almighty One (God), who banished Urd to Earth as punishment. Urd remained living with Keiichi, Belldandy and Skuld, not out of obligation, but because she liked living on Earth.
At the end of the story, Urd stands up to Lind, saying that she'll remain an angel no matter what happens,and returns to Heaven, her angel half ovrcoming her demon half (meaning she's considered a full angel rather than the Antichrist).
Urd is coy and snarky, but has a good head on her shoulders and always give Keiichi the advice he needs, acting as a mentor of sorts to Keiichi.
Urd has a rivalry of sorts with Skuld, based on the fact that Urd encourages Keiichi being affectionate towards Belldandy, something that Skuld doesn't like.
Urd is very proud of her gorgeous body and has a flirtateous streak.