Based on this and clips/headcanons from episodes and the trailer.
He swore so many times in his head, to his mirrors, and to even her face, that he would always protect her. That he would always keep her out of harm. That no one would ever take her away from him. And he was going to carry that promise even past his grave.
They broke into the city during the night. The explosions of fire were enough. But to hear her screaming, and then it fading too quickly, had enough for him to spring to his feet and run to her.
"The Avatar only falls in love once."
For some reason that kept repeating in his head as he saw her draped over the man's shoulder like she was a rag doll. The other three were giving everything they had on him and his brother and the Sato heiress. Tenzin and Lin were also putting up one hell of a fight, but somewhere in-between his search for her again, the armless water bender had him cornered.
"Hmm…it seems the Avatar's taste in a spouse doesn't change. Always the pretty ones.", she spat at him.
"Where is she?!", he exclaimed.
"And I guess the temper passes on too.", she commented.
"WHERE IS SHE?!", he shouted louder.
"Now you're just annoying. Another quality all the others had."
"The Avatar only falls in love once."
The water bender tossed him aside like a sack. And then Zaheer came into his view. And he had her still draped over his shoulder.
"You know boy? I give you credit."
Mako tried to stand up and balance on his elbows.
"I'd do the same thing if the love of my life was in peril."
"The Avatar only falls in love once."
"And that's exactly why I have to do this."
And then it clicked in his head. The Avatar only fell in love once. With the same soul that vows to protect them over and over and over again.
"But don't worry. A guy like you, you'll love again. Another day Another girl. Another time."
That was just it. He wouldn't. And he didn't want to.
He had no idea what had come over him. Suddenly he felt he had the strength of an army. And yet it felt familiar. He had used his leg closet to the man, and kicked him out of his balance. He quickly grabbed her before she could even touch the ground, and cradled her in his arms. But he was greeted too soon by the man's anger, and the three other criminals.
"Give it up boy. As death took the rest of her lives, it'll take her again. As will we."
He could feel the sweat from the heat of the night and the terror that flowed through him from the surroundings fall down his face. It was over. It was four vs one. He stood no chance. He quickly looked down for what he thought would be his last glance at her, a picture began to form in his mind.
There was a woman lying in a bed. She had long think brown hair, and dark skin. And she was holding a pale baby in her arms. And then there was a tall man. Dressed in yellow and red robes, with air bending tattoos.
"How does Tenzin sound to you?", the woman asked the man sweetly as he sat next to her, brushing the head of the child.
"Heh, sounds nice to me. But you picked the name the last two times.", the man laughed.
"I'm sorry. I forgot naming our first child after your lifelong friend was such an experience without you."
The man shook his head as he smiled.
"You know, sometimes I forget just how long I've been around.", the man said.
"Really? How so?", the woman asked.
"It's just…I've had so many past lives. And they all seem so short from what they say. But other times I feel like I've been around for longer than a hundred years. Because I feel like I've seen you hundreds of times. And I've fallen in love with you hundreds of times."
The woman's gaze softened.
"I love falling for you a hundred times."
"Then your future lives can struggle with the awkward flirting a hundred times more."
"We've practically mastered it."
His sight returned to Korra. He knew exactly who the couple was. And he knew he wanted the exact same thing. With her. And only her. Because it was never gonna get old. Even after hundreds of times. He wanted to do it a hundred times more.
"No. You're NOT taking her."