Marller was once the childhood friend of the goddesses who are the main characters, though now she's a demon. Urd and Mara used to be very good friends, but got separated when Urd chose to be a goddess.

Though Mara is the arch-rival of the goddesses she likes being with them. Her actual duty is to increase the market share of the demons on earth, her main duty should be signing demonic contracts with humans and her side duties includes ruining the activities of the goddesses, disrupting the efforts of the goddesses and to make them suffer. However, she normally forgets about her main duty and only goes around doing the side duties because she had decided in the very beginning of her appearance that the market share will become 100% when the goddesses are all gone from this part of the market.

Mara has two weaknesses: She can't stand good luck charms and she can't stop dancing when she hears rock music (or disco in the Dark Horse English translation).

Marrler seems more determined and mischievous than fully cruel. She felt abandoned when Urd chose to become a goddess in full, knowing that it would most likely lead to her having to end their friendship. This past situation and her duties as a demon make her have a slight case of tunnel vision in that she seems to regard ruining the lives of the goddesses a top priority. Despite this she does seem to enjoy the company of the goddesses whenever the opportunity to hang out with them presents itself.

In the TV series, Marller is revealed to suffer from insecurity and self doubt due to her inability to increase the demons' market share (due in no small part to the goddesses' presence), to the point of bringing her to tears when Urd tells her that she has no talent.

You may see Mara referred to by a number of other names, such as Marller or Marla. Marller is how Mr. Fujishima romanizes her name whenever her name is written out in roman characters, and Marla is how it sounds like when a Japanese speaker pronounces it and is also the romanization used by Geneon Animation for the Mini-Goddess series. In Buddhism Mara is also the name of a demon who tempted Gautama Buddha by trying to seduce him with the vision of beautiful women. Similarly in the series, Mara uses such tactics on Keiichi in her first appearance by using Sayoko to distract him from Belldandy. The reason most Japanese speakers prefer not to pronounce her name Mara is because the word 陰茎, romanized "mara", is Japanese for "penis".

(Source: Wikipedia)