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Batman has always had a tentative relationship with Catwoman, the only one that really understands him and vice versa. The main reason of that it's due to both of them having an alterego behind their true identity.

BATMAN: HUSH
*1She’s a criminal but one he feels drawn to. He decides that with all the madness happening in Gotham, he needs to be honest with her. So he takes her to a secluded place and reveals his true identity to her. While their relationship doesn’t work out in the end in this story, they prove to each other again and again that they truly love each other.

“Like you, I have two lives. I want you to be part of both of them.”

BATMAN: ARKHAM CITY
Catwoman has been locked up in Arkham City with the rest of Gotham’s biggest criminals. She ends up with a big choice. She could either leave Arkham City with the loot she’s been trying to get the entire night. Or, she could help save Batman who was trapped under rubble. Though she’s upset at herself, she goes and saves Batman. She knows in her heart she’d never forgive herself.

*2“You’ll never forgive yourself, Selina; help Batman, then escape.”

Leon felt in many times that he could approach Ada in a more intimate way. Always protective of her, and yet, so blind of her greedy goals. Ada also developed genuine feelings for him during their journey together through the years.

RESIDENT EVIL 2 (classic)
Ada Wong is a very talented assassin/agent,no one really knows who she works for at any one given moment, One minute she is the fbi, the next shes a spy for umbrella corporation. Leon and Ada however, since their first meeting, transpire about their characteristics to each other. He also jumped in front of a bullet, getting shot in the shoulder to save her life. And his words of kindness and humanity has made her heart melt, driving her to kiss him at their way to the rabbit hole (umbrella labs, full of monsters).
When Leon realizes she manipulated and lied to him about her identity and mission, he goes after her. She was doing her job as a mercenary, while he was doing his as a cop. Ada grabs her gun and demmands Leon to hand over the sample she was after, but doesn't take much until she lower the gun. After all, Leon was too good of a person to be killed, and she knows it. The place begins to collapse and she slips to the edge of a cliff but Leon's got her hand. Ashamed of her acts she let it go and disappear by the darkness. Leon feels empty inside, due to the growth of feelings he developed for her.

RESIDENT EVIL 6
On the elevator ride up to the roof, Leon is quiet. Helena breaks the silence, commenting,

“She’s more than just a friend, isn’t she? You have feelings for her.”
Leon says nothing, though gives a small smile.

RESIDENT EVIL 4
At this time their path keeps getting crossed over and over. While Leon tries to contain a sample of a dangerous virus, Ada seeks to sell it. Even if Good VS Bad is at stake, their past together still means alot to them both. So when Leon is in danger, Ada throws a paper airplane with her signature red lipstick kiss into the cell room Leon is just about to leave, saying:

“Perhaps you have figured it out already, but you might be able to get out of here by using the waste disposal vent.”

*1Selina Kyle is a criminal, Ada Wong is a mercenary. Both of them doesn't really cares about right and wrong, good or bad. It's all a matter of survival.
As for Leon and Batman, they both wants to make the world a better place, no matter what.
*2Leon and Batman, Ada and Catwoman, they all gets gullible between their relationship during the time they spend together, wanting it or not. The ultimate element that makes this connection with the two universes, is how a woman can deeply affect the mind of a man by giving him a reason. And how a man can deeply affect the mind of a woman by giving her the meaning.

All in all, both couples are under the spotlight of the best thrilling and romantic experience that every comics, movies or games has ever done yet.