Every Anime has it's bad guy, and Ah! My Goddess is no different. Although she makes no appearence in the OAVs, except for the intro theme, Mara plays a fairly large role in the Manga, and appears in the "Adventures of the Mini-Goddesses" series.
Mara is a demon from the Niflheim (evil equivelent to Yggdrasil) who is constantly trying to cause trouble for the Goddessess. In the Anime she seems to be the head-honcho demon who takes sheer joy in bothering Belldandy and her sisters. However, in the Manga she is a mere salary girl, who works for Urd's mother, Hild, who Mara is continuously trying to impress by tracking down people who've come in contact with goddesses and offering them a 'better contract' than the ones that goddesses offer.
Urd and Mara used to be best of friends. Apparently, they used to spend a lot of time together playing video games and singing karaoke. However, due to the natural conflict of their positions, they were forced to part ways, and ended up as 'enemies.'
At one point in the Manga, Mara ends up 'vacationing' with Keiichi and the Goddessess. While there, Mara and Urd play video games, sing karaoke, and act like a couple of silly girls at a slumber party. Although Urd and Mara fight sometimes, and their jobs leave them at a difference, they will always be the best of friends at heart.
Of course, whenever Urd goes balistik and turns 'evil,' you can usually count on Mara having something to do with it. The perfect example is during the "Terrible Master Urd" saga, when Mara is trying to re-awaken 'The Lord of Terror.' Ironically enough, Urd awakens as 'The Lord of Terror,' or rather a vessel for 'The Lord of Terror,' and Mara is forced to do her bidding, much to Mara's dismay.
Her main weaknesses include good luck charms and disco music. Whenever she hears disco music, she can't stop dancing, which is exploited by Urd on a number of occasions.
Mara is the series main baddie, but whenever she's around, you can always count on a big laugh.