Relationships
Part of what makes Mara such an interesting character is the way that she interacts with the rest of the cast. Mara is deeply conflicted about her feelings toward certain characters like Belldandy and Hild; with others, however, she's more straightforward. Keiichi can just turn into a worm for all that she cares. ^^ In case you haven't figured this out yet, this page here delves deeper into Mara's relationships with others, and analyzes her often complicated feelings for the most important people (or Goddesses, or demons) in her life.
-----> Belldandy
Being the heroine of the story, Belldandy is naturally Mara's greatest enemy. However, Belldandy herself is so kind and gentle that she's always willing to give Mara the benefit of the doubt. Make no mistake, Belldandy does get angry with Mara, and doesn't hesitate to strike out against her when she feels that her loved ones (read: Keiichi) are in danger. In chapter 27, Belldandy is even willing to use the Gods CD to seal up Mara for 500 years, something which she said earlier in the chapter was "a little extreme, even for a demon."
Belldandy and Mara were childhood friends, and because of their long history together, they still both know each other's personalities, weaknesses, and secrets. Belldandy senses that Mara is sometimes very lonely, and sometimes very sad, and for those reasons treats Mara with kindness when the situation allows her to. In chapter 29, when Mara is suffering the ill effects of stepping on a hamaya, Belldandy even gives Mara a hug and says, "Don't worry! I'm not going to abandon you." Despite the fact that Mara seems hell-bent on tormenting Belldandy, and despite the fact that Belldandy does not hesitate to defend herself with aggressive force, it is still clear that Belldandy feels a deep sympathy toward Mara. "Mara... I... Well, my job doesn't let me wish you luck. But I wish you well just the same," Belldandy says to Mara in chapter 60.
-----> Urd
"Mara always was a nasty little bugger, even as a kid," Urd says of Mara in chapter 27. Like Belldandy, Urd was also a childhood friend of Mara. In fact, the friendship between Urd and Mara extended well into their adulthood, as the events of chapter 60 reveal. Urd and Mara used to go out drinking together all the time, before they got their respective jobs of Goddess and demon. (Although even during that time, the two of them played vicious pranks on each other, such as slipping bugs and poison into each other's drinks.) Mara and Urd still like each other, as Belldandy observes. And Mara is frustrated that she can no longer be as close to Urd as she would like to be. "Damn it! If you weren't a Goddess... And I wasn't a demon... We could hang out together like this every day!" she says to Urd in chapter 60.
There is some implication that Mara resents the fact that Urd, a half-blood, chose to become a Goddess instead of a demon. Mara is drawn to Urd's darker side, and obviously misses the companionship of her old friend. Which brings to mind the question: why did Mara eventually capture Urd and forcefully separate her Goddess and demon halves, creating a second Urd who is 100% demon? Mara acts as though her plan is motivated by just wanting to bring trouble upon the Goddesses. However, one could examine Mara's actions more carefully, and conclude that she was really trying to "bring back" the Urd who was her old friend, or at least create another version of Urd who could still be her friend. In chapter 64, after Demon Urd has gone berserk, Mara abandons her dignity and runs to the Goddesses, and begs them to save both halves of Urd. "I can't lose you," Mara tells Urd's good half. "I still haven't beat you at Tetris, dammit..."
Despite the fact that Mara truly does care deeply about Urd, she usually doesn't let that get in the way of the fact that, as a demon, it's her job to torment a Goddess like Urd. Mara never hesitates to attack Urd with polka music (which makes her fall asleep), or to gloat whenever Urd is suffering. "What a scream!" Mara says to herself when she hears that Urd has to leave Belldandy and Skuld, and return to Heaven (chapter 47). "I've been through a lot of grief because of that broad. This couldn't happen to a nicer person! Take that, Urd!" Yes, Mara is still at heart a nasty bugger, even where her friends are concerned. ^^;; Although as a general rule, since Mara spends most of her time focusing on Belldandy, she rarely ever directly harms Urd, although she does not hesitate to do so when Urd tries to interfere with her plans.
-----> Hild
Hild is Mara's boss, and she obviously enjoys ordering around her underling demon. Hild addresses Mara as "Maa-chan," but knowing Hild, this could just as easily be a veiled insult as it is a term of endearment. Hild does a couple of awful things to Mara, such as striking her with a lightning bolt, or ordering her to "demonstrate" the Soujinki by using it on herself. However, no matter how nastily Hild treats Mara, she always does so with a smile and a giggle. From Hild's point of view, she's not doing anything more than harmless joking, albeit most of her jests are actually quite sadistic. Maybe Hild just has a very weird way of showing Mara that she likes her. Or maybe Hild is just evil incarnate. It's open to interpretation.
If it's hard to discern the way that Hild really feels about Mara, it's even harder to figure out how Mara feels toward Hild. During some parts of the manga, she looks up to Hild, and seems to admire her knowledge and her power. At other times, Mara cringes before Hild like the terrified subordinate that she is. And at still other times, like in chapters 140-141, she works alongside Hild as an equal. Finally, during the Rind story arc, Mara shows yet another side to her view of Hild - she is horrified by the depths of Hild's depravity. In chapter 159, after Hild explains the true goal of her plan involving sending the Angel Eater after the Goddesses, Mara stares at Hild with an expression of disbelief on her face, and thinks, "This really is the leader of Hell... Like a spider's web... Being trapped and unable to move..." This last line corresponds with Mara recalling a mental image of Belldandy tangled in a spider's web. However, the meaning of Mara's soliloquy is left ambiguous. She could just as easily be referring to Belldandy caught in the web of Hild's plot, or herself, left with no choice but to obey Hild, or perhaps the both of them together. Despite whatever ethical reservations she may have, Mara goes along willingly with Hild's plan, and never again shows even a hint of reservation. But then again, in chapter 160, Hild not-so-subtly reminds Mara that she can read her mind. So whether Mara agrees with Hild or not, she acts (and thinks) as though she does anyway.
-----> Keiichi
Keiichi is Mara's favorite victim, simply by virtue of being Belldandy's loved one. I won't go into all the details here, but there are other pages on this site where you can find listed all the awful things that Mara has done to Keiichi. After being so often tortured by Mara's evil plots, Keiichi finds it hard to believe that Belldandy or Urd ever once liked Mara.
In the early part of the manga, Keiichi also spent something like three chapters laboring under the mistaken impression that Mara was a man. Mara decided to play up on that, and in chapter 28, shocked Keiichi by telling him, "Belldandy's my fianceé." Both Mara and Urd got a good laugh out of that one in the end.
-----> Skuld
Mara rarely, if ever, directly interacts with the youngest Goddess. However, since Skuld built Banpei, the "anti-Mara tactical strike robot," there's no love lost between them. Banpei is armed with rocket launchers that shoot out good-luck charms; during his first encounter with Mara, he succeeds in literally blowing her out of the sky. So yeah, definitely no love lost between them. Since Skuld is much younger than the other Goddesses, it is unlikely that she knew Mara as a childhood friend, and thus Skuld sees Mara as nothing more than a pesky, annoying demon.