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)

Keiichi Morisato is looking forward to university life. But in reality, he has no luck in anything, and he has trouble with clubs, love, etc. The truth is that he has an unlucky star above his head. One day, Keiichi is stuck watching the dorm while his sempai are away, and has a mountain of chores to do to boot. But Keiichi is a good-natured person, and is set about doing his duties. As he is about to finish his final chore, he makes a phone call to his sempai. But the words that came through the receiver are, 'Goddess Help Line.' Shortly afterwards, a beautiful goddess named Belldandy appears in front of him from the mirror of his room. 

Alone in his dorm on a Saturday night, Nekomi Tech’s Keiichi Morisato dials a wrong number that will change his life forever—reaching the Goddess Technical Help Line. Granted one wish by the charming young goddess Belldandy—a wish for anything in the world—Keiichi wishes she would stay with him always! Complications are bound to ensue from this; the immediate first being the new couple getting tossed out of the dorm—it’s males only! 
As the hapless student and his mysterious “foreign beauty” ride around looking for a new place to stay—risking the different dangers of seeking shelter with an otaku convinced Belldandy is an imaginary woman, and a Zen priest convinced she’s a sinister witch—Keiichi’s still got his classes on Monday morning! How is his new “exchange student” companion going to be received on the N.I.T. campus? A little too well for normal life to ever return... (Source: Dark Horse) 
Note: Won the Kodansha Manga Award for General Manga in 2009.

The song may say "thank heaven for little girls", but who is college student Keiichi Morisato supposed to thank when three honest to goodness Goddesses move in and take over his life? After all, according to most mythologies, relationships between mortals and immortals usually leads to mortifying complications, so we're already talking about an extremely mixed blessing!
 
(Source: Crunchyroll)

An Angel Eater has been freed in Heaven. Since Goddesses and Angels share body and soul, losing your angel results in going into shock for the Goddess. Lind goes to Earth to protect Belldandy and the others unknowing that it is a trap.
 
(Source: Anime News Network)

A large collection consisting of the adventures of the Goddesses featured in the anime and manga series Ah My Goddess. Parodies of other works, and a large number of jokes pervade this series of shorts in which the Goddesses torture and hang out with their friend Gan the rat.

In the story, Keiichi Morisato and Belldandy lived in happiness, but due to the recession, the motorcycle shop Keiichi works at faces financial difficulties. Belldandy starts to search for a job because of the situation.
 
(Source: Anime News Network)