Why did you do it?
It wasn't right what happened to you. I thought I could help.
Help? You wanted to help me?
I wanted to tell you it wasn't personal. I didn't think you should spend your life crippled because you were in the wrong place at the wrong time.
Before leaving Patch, Yang sent a letter to General Ironwood in Atlas, thanking him for the gift. Whatever differences of opinion they had, Yang still wanted to express her gratitude for him giving her something back.
When his reply found her she was already staying in a guest house in Mistral with her sister and their friends from Beacon, and was quite surprised to learn that Ironwood hadn't commissioned the arm as she believed; he'd merely paid the fees to ship it to Yang. He'd received it anonymously, and after his techs verified its authenticity, he moved it along to her.
Yang didn't think much of it at the time, but as she was working hard to train with the others and became accustomed to compensating for the different firing rates between her metal arm and Ember Celica, she started to wonder who'd put in the effort (and the money) to give her something like this.
Whoever it was had been familiar with her fighting style. Maybe they'd attended Beacon or Signal... or watched all her rounds in the tournament and observed her keenly enough.
But it was an exceptional gift, and Yang started to wonder why it'd been given to her.
You never had to do what you did. You never had to end up here.
We all got our choices to make. You made yours and I made mine.
So why help me if we weren't on the same side? What do you get from this?
What I want most of all: you at your best. And me at my best.
The attack on Haven was on the horizon, but a few weeks beforehand Qrow received a request for help from the most unexpected of places: Raven and her bandits. As Qrow was trying to parley some assistance from his sister and her tribe anyway he thought by rendering them aid they might be amenable to joining the fight against Salem.
Yang wasn't eager to see her mother again, but she certainly agreed that Salem was their common enemy. So she went along.
Once there, finding the bandit camp set aflame, Yang thought back to the Battle of Beacon. For a brief moment she doubted herself, as she recalled her failure then... her loss then.
But Yang was fighting beside her family now and for her family now. She wouldn't let the past control her. She wasn't going to let fear compromise who she was. Not again.
Qrow was directing traffic, but Yang saw her opponent step out to confront her before he'd issued an order. She saw him stare her down and knew she could fight no one else.
"This one's mine," Yang growled. No one challenged her assertion.
He skidded his metal foot along the dirt and smirked at her. Yang reared back her fists and started to approach while Ruby, Qrow, and the others went to help Raven and her Spring Maiden. The silver-haired foe standing in Yang's path whimsically asked her: "You enjoying the gift I sent you, blondie?"
Yang stopped right there. She took her eye off him -however briefly- to look down at her right arm. "You... you sent this to me?"
Mercury only smirked, thumbing his nose.
"Why did you do it?" Yang asked, genuinely curious.
"It wasn't right what happened to you," Mercury mused. "I thought I could help."
"Help?" Yang incredulously repeated. "You wanted to help me?"
"I wanted to tell you it wasn't personal," Mercury explained. "I didn't think you should spend your life crippled because you were in the wrong place at the wrong time." He pointedly raised one of his legs and tapped his artificial shin. "Or because you picked the wrong fight."
Yang glanced around at the fires raging, and thought back to Beacon and the fall. "You never had to do what you did," Yang fiercely reminded him. "You never had to end up here."
Mercury shrugged. "We all got our choices to make." He then pointed at her. "You made yours'." Then he resumed his combat stance. "And I made mine."
Yang still couldn't make sense of his motives: "So why help me if we weren't on the same side? What do you get from this?"
"What I want most of all," Mercury explained. "You at your best. And me... at my best."
He threw their fight in the tournament. He let Yang believe she'd beaten him; let her believe she was the better fighter, and he thought he knew better. He wanted Yang to know it. He wanted Yang to feel the defeat he'd willingly endured at his master's order.
Yang understood that. She didn't know if it merited the expense he'd invested into Yang... into restoring an enemy's strength just so he could fight her again. But she knew the desire to fight. She knew the desire to prove a point.
It wasn't a rational wish, but Yang understood it. This wasn't about Yang's family obligations or Mercury's duty to his master. This was about them. This was their fight, even if it was just one battle in a much larger conflict. It was their score to settle.
"I promise not to go easy on you," Yang assured him.
Mercury smiled. "Wouldn't have it any other way."
Yang moved in and her new fist met his old metal leg, one final time.