My mc poses some strange shadow powers which he tries to hide and in one quest he met his enemy who also has these powers. They both end up fighting each other and the mc slows her down and escapes from there. Then he realizes that they both were in the same city working in the same place so they end up fighting again and also the enemy is a childlike, funny person who masks her deepest feeling (sadness and isolation) and she hides this personality and always be angry towards mc (only to mc). How can I make them both fall in love or attest mc fall in love with her or at least his enemy fall in love with him without making the readers bored and also to avoid cliches?
If the female enemy is immature, make the male be emotionally more of an adult, even though they are about the same age. He is merciful; despite her apparent anger or hatred. Invent a situation in which he captures her or disables her power, but instead of harming her, lets her go. Then another incident, she attacks him. He could hurt her, but he doesn't. Lets her go again.
The point of this is you want your female to owe him; he never claims he is doing her a favor but she knows he did. He begins to interest her. She starts spying on him.
Let her see him actually harming other enemies; so she knows he isn't afraid to do that -- he just wouldn't do it with her. You want her to see that he isn't a normal enemy.
While she spies on him, she sees him get trapped by a different enemy; and he isn't going to get away. She steps in and saves him. Or maybe just releases him and he saves himself. But he knows it was her.
But this little quid-pro-quo starts a kind if rivalry/friendship, she doesn't want to battle him. She has a lot in common with him, and it isn't long before they team up: Together their common powers make them stronger as a team: Even without any magic two people can get more accomplished with teamwork than the sum of what they can do alone. They can have each other's backs.
Then the camaraderie of this "working together friendship" becomes inevitably personal, they like each other, and look forward to being together. Give her a personality trait he likes (perhaps her humor), give him a personality trait she likes (maybe he is smart and insightful and just understands her better than anybody else has, he gets her feeling of isolation and loneliness.
And maybe he feels the same, so just companionship with somebody like themselves is an important value for both of them.
You want them to become co-dependent, both happier and more successful together than they are separately. Then it is a small step to physical attraction, and turning their friendship into romantic love. Easier if they are both thinking the same thing.
How does love work? It's strange. It needs attraction, it has chemistry and most certainly, it does not care who is friend or foe.
The mere fact that they fall in love is not a problem. The real question you should ask yourself is: How to write it in a way that your readers don't go onto the barricade demanding you to burn the story on a stake?
There are LOTS of ways to do this. Let's take a look at some tropes that deal with this (tvtropes warning!):
- Foe Yay
- ~ Shipping - This is when the READERS decide that the two belong together... despite the Author claiming the opposite.
- Foe Romance Subtext - Their relationship is more implied, sometimes crosses over into the Villainous Crush. If they make it into text, see Dating Catwoman.
- Dating Catwoman - They go out while still being opposed sides. It puts stress onto the relationship in strange ways. Catwoman & Batman are an obvious example. Can lead to Defecting for Love.
- Slap-Slap-Kiss - The instant type of fighting and getting the protagonists to kiss for one reason or another. Can (in some stories) lead to Destructo Nookie. Without kiss it's Belligerent Sexual Tension.
- Love at first punch - Similar to the kiss, love from fighting.
- Villainous Crush - For some reason, the antagonist opposes the protagonist because they have a crush on them. Some depictions of the Joker are this to Batman. See also Stalker with a crush, Villainess wants Hero, Mind Game Ship and In love with your Carnage. Can lead to Dating Catwoman and further.
- Defecting for Love - When the love leads to redeeming/corrupting one of the two.
It really isn't that hard. It is universally known that love is a mysterious thing that defies logic and requires no explanation.
Virtually every serial killer on death row has a small cadre of admirers of the opposite sex. As the vast majority of serial killers on death row are men, out of a population of 330 million in the US of A, there are plenty of women who are into the death row bad men.
Then there was Napoleon Bonaparte who bemoaned the fact that while he almost became the master of Europe, he never once found romantic love.
So don't fret too much about explaining love, or why two people are or are not in love.
Just write the story you want to write.
George R. R. Martin never bothered to explain WHY Jamie Lannister Loved his sister, with so many available options. He just does. And the readers accept it.
Haters are going to hate. And whatever you write, haters will declare it cliche. But not everyone is a hater.
And trust me, people won't like you more for providing a laundry list of why.