Love and RelationshipEdit
In the movie, one scene may define their feelings for each other. Belldandy expresses that "there's an ugly part inside of me that wants your love for nobody but myself." Keiichi responds by telling her that "it's not despite that, but because of that, that I love you."
There are many defining points in the manga continuity where the two have to constantly reassure each other that they love each other. Keiichi wishes for Belldandy to be with him not because of the contract, while Belldandy is always telling Keiichi "as long as you need me, I'll be by your side."
One major point of confession is after the Lord of Terror arc, when Belldandy tells Keiichi that she's not going to Heaven. She didn't stay on Earth simply because of the contract (because a contract by itself cannot bind a goddess against her will). This is reaffirmed in Peorth's Fourth Goddess arc, where in the end Belldandy openly states that she's on Earth because of her love for Keiichi.
How their relationship works? That again can be defined by the movie. In the movie, the racing bike that they use is an altered motor vehicle with a passenger placement. The driver has to place all of his faith into the passenger and pull the throttle, because the passenger is just as important as the driver; the racing bike can go either faster or slower due to the passenger's actions. It's much like when they passed through the Judgement Gate. Although they have their fears, they aren't afraid to trust in one another.
Keiichi loves Belldandy because of her kind and gentle qualities, as well as the fact that Belldandy is rather possessive. Belldandy loves Keiichi because Keiichi has many great qualities that she sees in him, and she helps bring them out in him as the series progresses. Also, Belldandy is under a great deal of pressure to be perfect, as she is a Goddess First Class. Keiichi unconditionally accepts her and whatever faults she may possess, and that is probably another reason why Belldandy is so willing to stay with Keiichi until life ends. She wants that unconditional love and affirmation.
Development in the Different IncarnationsEdit
The OVA is basically a service to the fans of the manga, so a lot of their relational development was skipped. However, a new factor has been put in: the ring. The memories of the past as well as the sacrifices they had to make add on a new layer to their love.
The TV series originally had the same idea as the manga (to develop the relationship slowly), and sort of accomplishes it. In the end, though, the rush to get in the other characters and such caused them to neglect some key moments (such as the ring scenario). However, this is more than made up by the ending, where in the second to last episode had Belldandy telling Keiichi that a world without him doesn't matter to her, and in the last episode where Keiichi tells Belldandy that he's glad that she's decided to stay by his side.
The manga drags it out. Period. According to the Three Ninja in Ah! My Goddess Colors, "it's a neverending irritating love story."
The future at this point is very vague. My own personal speculation is that since everybody knows that their relationship is unnatural, there will come a point in time when Heaven cannot afford to have the Norns leave Yggdrasil unattended, and will have to recall all three goddesses because of a possible future apocalyptic event (possibly called Ragnarok or Gotterdamarung, since Oh My Goddess draws a great deal from Norse Mythology). This will override the contract between the two, and my own personal guess is that Belldandy's goddess identity will be eventually stripped away because of this apocalyptic battle, and will become human. On the other hand, Keiichi may be raised to a similar level of existence as Belldandy, possibly involving an appearance of Valhalla, where fallen warriors and heroes became similar to gods.Then again, it looks like Mr. Fujishima has little to no real end in sight due to him dragging on the series and publishing it at an irregular interval, so...