When it comes to the love interest, the big problem is the power issue. In essence, the love interest is either more powerful or less powerful than the main character. This is a mistake.
Consider the TV series “Charmed” for a moment. As the series progressed, the power level of Leo (Piper's love interest) rose, hit a plateau, and then disappeared. He began with the power to teleport (in the series it was called “orbing”, but let's call a spade a spade, okay?) and heal. He then became an Avatar, becoming practically omnipotent, and then lost his powers completely. He still had his knowledge, but he was pretty much secondary to his wife. Although he was still a great character, it felt as if the character had been emasculated. Worse, what made him important to the team was taken away and his involvement in the team became more passive.
Although an interesting feminist statement, part of what made the Piper-Leo coupling interesting was that Leo was Piper's White Knight, the man who would come to Piper's defense no matter what. However, as the series progressed, Piper became more independent of Leo; this was great on the level that women shouldn't be dependent on the male in their life, but we had already had that in the season after the Avatar arc. However, Piper also tended to abuse Leo a lot; in a real-world marriage, they would have been forced into marriage counseling by The Elders early on. By diminishing Leo's power, they not only forewent making an even more important statement (that marriage partners should be equal), while at the same time creating a third-rate character.
At the same time, consider Tuxedo Mask from Sailor Moon. Because Tuxedo Mask was more powerful than Sailor Moon, it was easy for her to come to depend on his timely arrival, while resenting him if he didn't rescue her. In essence, she may have received top billing, but she was a secondary character every time he showed up.
The power level issue is one that needs to be seriously considered; after all, if the lover has a lower power level than the loved, then the relationship has a built-in conflict: Is the lover in an actual relationship, or is he or she just a pet? This is most obvious in super-hero and vampire stories; what does Lois Lane offer Superman? Worse, with vampires you have the problem that the current lover merely reminds the vampire of past days and his humanity; the vampire isn't necessarily in love with the person so much as the idea of being in love. The greater the difference in power, the more you need to justify the relationship.
So, how do you justify the relationship? You don't. Once you start justifying the relationship, you finish it; by exploring it, you remove the mystery that is inherent in any good relationship. Instead, go with the romance; a few surprise gifts, making sure that the lover is called no matter where the call comes from, and giving the lover some space. If necessary, give the lover a signal device or bodyguard, but she should have something in case she gets in trouble.
Now, if you think the romance is fun, imagine what happens when the romance is over! You have someone who knows the loved one's secrets, limits, and psychology; that could be one heck of an enemy...