People love to compare Roy and Bradley or Ling and Wrath, but I think there is a special beauty in the narrative parallels between Riza and Winry.
Because these woman manage to break out of the Shounen mold in a way that reaches beyond the Strong Female Character trope and quickly secures itself as... good character writing. Period.
And they do so in a very clever way. Someone else on here once pointed out that Hughes/Mustang/Hawkeye are the trio that runs parallel to Ed/Al/Winry and while they aren't narrative foils to each other - at least not in the way many of the other characters are - they do present a similar function within the story. The three young people who went on an adventure. Only Hughes died and Riza and Roy were permanently altered - and Ed, Al, and Winry got a chance to save the world.
But especially when it comes to Riza and Winry there is something more to the comparison. Especially when it comes down to the choices they made.
But why are Riza and Winry more interesting?
Because when Roy recruits Ed and Al, Riza tells Winry that she followed Roy into the military because she had someone to protect - and this - in other stories - would clearly be a setup for Winry later following Ed and Al into the military to "protect" them. A direct parallel between the two "girls" in a Shounen trio. We've all seen it before.
And I think we see Winry play with that thought when she sticks around Central with them after her first apprenticeship in Rush Valley - she tries to be the third girl to Ed and Al's action duo… but it doesn't work out.
She - strong, clever, genius, confidant Winry Rockbell - suddenly feels weak. Because she can't punch danger away from Ed. She can't repair Al's scratches and dents. She can't kill Scar to save her friends and avenge her family. She can't learn how to shoot and kill just to protect her friends - no, that's not quite correct, is it? She won't. She won't learn how to kill.
And that sucks. Because Winry isn't used to feeling like that, so lost and insecure, at least not constantly. Yeah, when Ed and Al are away, she worries, and that's part of the reason why she tried to join in, but that is nothing compared to the powerlessness she's facing now. Maybe she would worry less if she could be there when they fight, if she could protect them like Riza does with Mustang… but that's just not who she is.
Her job isn't to protect Ed and Al - her job is to give people arms and legs and good costumer service. I really like that scene/episode (23, me thinks) where she gets a phone call from Rush Valley and all these people ask for her to come back. Because Yes, Ed telling her thanks for helping him is VERY important for her character… and yet I think this phone call is the moment Winry realizes that she's not Riza. That she won't take a gun into her hands and kill for Ed and Al.
She will never be Armstrong or Hawkeye or even Izumi… she will be Winry Rockbell, automail engineer and genius.
And that's the reason why only she could have pulled Scar on their side. Because she chose healing over killing - her telling Scar in Baschool that she'd save his life because her parents would want her to honor their choice? That was Winry following the deeper themes of the show, by adding positive energy to the flow of the universe.
Riza saving Scar? Wouldn't have worked (why would he listen to the woman with a gun in her hands?). Armstrong helping Scar? Wouldn't have happened (what reason would General Armstrong have at this point to spare a murderer?). Mei saving Scar? Would have ended with the Ed/Marcoh/Scar/Al alliance falling apart (it is so much easier to fall apart if no one has been forced to see past the horror yet).
And it's not because these characters were even a touch less well written than Winry - if anything it showcases how unique all of the female characters in FMA were/are.
In this we find Riza again - because Riza chose differently than Winry. She followed Roy into the military, she learned and perfected how to shoot and kill. Their narratives mirror each other - Ed carefully prying a gun out of Winry's hands so she doesn't kill, only to give Riza a bloody gun a few episodes later, knowing she will clean it and use it to kill.
When Riza tells us that she has lost the right to feel squeamish about killing because of often she'd pulled the trigger, she is Winry's foil - Winry who was stopped before she could make a similar choice.
And it's not just that, is it?
Riza let her hair grow because a young Winry Rockbell had long hair and seemed to like it - and Riza needed a change after coming back from Ishval.
Winry got her ears pierced because the strong Lieutenant visiting them had looked cool (and because she needed a place for all of Ed's little gifts) - and Winry needed something steadfast, now that her friends were growing up.
There's just something about the two of them, so similar, so loyal and stubborn and full of love, that fascinates me. Because at every turn they make a different choice, at each turn one walks deeper into hell and the other chooses healing - and yet, while they couldn't be more different, they also couldn't be any closer.
I can't imagine how glad Riza was, when she realized Winry hadn't followed Ed and Al into the military.
I can only guess how happy Winry was, when she saw Riza follow Mustang further if only to make sure the future actually changed.
A mirror doesn't have to be a perfect thing, and if anything I think that is on purpose.