It's time to talk about Riza OR here's what I think about her decision to join the military.
I don't like the theory that Riza joined the military JUST FOR ROY or rather, to protect him. Meta autonomy points aside (I just don't like the idea of her entire life revolving around first her father then Roy, as much agency as she obviously has within the text), I'm gonna talk about what I think about Riza's journey and why I think she joined the military.
We are offered two reasons as to why she joined the military: the reason she gives Winry "there's someone I have to protect", and the reason she gives Roy (and talks about later with Ed, I think) which is "I joined the military to help people".
And here's what I think about which reason is more prominent, or rather, which was her original motivation. (Clearly both are important motivations for her.) And I'm gonna go into why I think they're interlinked, too.
Riza was definitely in an oppressive, abusive situation with her father. I don't think he outright physically abused her, apart from the tattoo thing (though it's certainly possible/probable), but if he's prepared to treat her as nothing more than an object upon which to record his research, there's no way that was a good living situation to grow up in. I mean, the reason Master Hawkeye gives Roy to protect Riza (nicely subverted by Arakawa having Riza being the one protecting Roy later, btw) isn't because she's a good person or, hell, because she's his daughter, but because she's "in possession of my research". Hello bad father of the year, jfc.
And then Roy came along and he was everything Riza's father wasn't. And he talks about this idealistic dream he has for reforming the country. She's so cynical even at such a young age, because she was robbed of her childhood and forced to serve her father. (God Arakawa knows what this did to her psychologically - she must have grown up not learning to appreciate her own identity, without encouragement or emotional support.)
So she hears Roy talk about his dreams and it has a profound effect on her. She'd never cared about the country before, but now she kinda sees the situation as a parallel to her own life, and she wants to help reform the system to help those oppressed like her. (I mean, she may not be aware that this is why she's drawn so much to Roy's ambition, but it's definitely there subconciously at least.) Also her father hates the military, that's canon. So Riza becoming part of it is her way of completely separating from him, punishing him for what he did for her, taking her life back.
But I believe she's primarily motivated by a true desire to help people - this is what Roy inspires in her. And here's my proof of why I think this is her main motivation in joining the military.
The whole protection theory only works if SHE KNOWS SHE'S GOING TO BE ASSIGNED TO HIM.
But there's no way she could have known this! And it's already a good while into the war, and years since the meeting at the grave, when Roy and Riza have their reunion in Ishval.
So this is my proof that she joined the military for her own reasons. Then, in Ishval, while so many soldiers were happy with their jobs, or at least wouldn't voice their opposition to the extermination, Roy openly talked about how wrong it was. Even though he still followed orders, Riza knew the best chance to right the wrongs she committed was to stick with him, to support his rise to power so he could change things and atone for what they'd done. And he needed someone to protect him. So she carried on through her grief and guilt (and, imo, self loathing) to work with him for reform and redemption.
Basically, Riza protects Roy both because she cares for him and to fulfill her original motivation of helping people.
But initially I believe she joined the military to help people, and although Roy inspired her to do this, she wasn't joining for him, or in the hope that she'd see him again so she could protect him.
