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.