I would like some advise how to write these two characters in this story I would like to write. Or more on their relationship. At first the main character despises this character (I hate calling them main character and this character so the mc will be Ann and the other will be Phil) Phil also dislikes Ann back untill a tramatic event that cause both to lose their memory of each other and for 5 years they never meet untill they are abducted by and evil woman that I'll call Red for now. At this point when Ann's friend (who was at the tramatic event but did not lose her memory and also knew them before, I'll call her Emily) tells them about their horrible past their hatred grows for each other once again. But they have to work together (plus with some other characters) to stop Red. Over time as they try hard to defeit the evil Red, Phil begins to fall in love with Ann. But it takes Ann longer with more trials to do the same. You see I have no experience writing love stories and I'm better at writing fantasy and sci-fi (in fact this is more of a fantasy, it takes place in a mythical world and Red is a witch) I would like advice on what changes their hearts to love instead of despise. And how fast should their aditude change should it be slowly or like a surprise. Take note I didn't use their names, if that wasn't obvious enough
Maybe they both see that they were lucky to survive the first experience and with that they become slightly closer but still are not fond of one another but the longer they are together and the more they go through they both see that in the grand scheme of life their disagreements aren't as serious as they once thought and with that they slowly become closer and develop a better relationship... Just a thought. I hope it helps
I agree with Raging_Ty, essentially, having them go through perils and help each other out, they can realize that their previous relationship may not have been how they really felt. You can use many movies/books for reference (basically anything that has the male lead and female lead bickering to begin with but end up falling in love by the end of an action packed adventure)
In terms of writing, my main advice would be to show it occuring than telling the reader that it's happening. I'd show the changes happening gradually in order to make full use of the conflict... and yes, I agree with the last two posters, working together to achieve a mutual goal is great for building relationships. Bickering is also always a good way to go. Other than that, just write it and see how it goes from there...
Since your characters don't actually remember hating one another in the past, maybe they can lose trust in the person who told them about their past, and begin to doubt everything they believe to be true? It may make them more open to forming their own, new opinions about one another.
They say opposites attract. Often it's those very things that allows your personality to complement your partners' that causes trouble. As they figure out how to enjoy their differences love follows. Also hate is much closer to love than indifference. And a final thought, who can explain love. It just happens and it usually doesn't make sense.
Why don't they naturally start to hate each other even after the memory loss? If it's a matter of personality, annoying habits etc, memory loss wouldn't affect things because they'll grow again. But if they find that they get on just fine once they've forgotten the past, then I'd question why they suddenly start to hate each other again after someone telling them a story? You'd question if it really happened, and you'd ask if it even matters (depending on the reasons behind why they hate each other). And if by the time they find out about their past, they've formed a friendship, they might talk it out, they might decide to forgive, they might give each other credit and simply say "Well they've changed for the better now because this is who I know now." They wouldn't just throw a friendship away based on something they don't even remember.
Generically when you have a relationship that starts from two characters having previously hated one another there's a moment when generally the girl does something the guy can't fault and so he respects her for it. And then when he's observed something in her he likes he'll start behaving better towards her she'll then fall for him. I just thought I'd point this out as a technique that has been used very effectively in the past as you might want to do something a bit different. But that said you do need a moment when one or both of the characters soften towards each other and there does need to be a reason for it otherwise it'll seem a bit random. If they're working towards defeating Red together I think that provides a nice reason in that they'll grow closer and bond together over the common goal and it'll be easier to show it through their actions rather than explicitly saying how they feel, if you prefer that approach. Anyways hope this wasn't too unclear and good luck.