"Of course I will, sir. I'd follow you into hell if you asked me to."
There was always something about Roy Mustang that drew Riza to him. Even before they had considered each other "friends", when they were in that awkward middle ground between casual acquaintance and actual friendship that was only heightened by how confusing and lopsided an experience being a teenager can be, Riza felt this undeniable need to be around him, to get answers about him and answer questions about herself that, when Roy asked them, she was all too eager to answer. At the time (and, to a point, even now), Riza wasn't able to pinpoint why she felt this way, why her interest in him seemed to flow as endlessly and powerful as a waterfall, accumulating at the bottom into a lake of new things Roy told her about him, his family, how he saw things, etc. It could have been that he was new to her and, like anyone with anything (or anyone) they've never come across before, Riza's curiosity knew no bounds or exhaustion. Of course, there was always the chance that it was his appearance physically that made Riza so interested, although she doubted that was the whole reason. Of course, it didn't hurt that Riza did find him handsome, as apparently many girls in town did. To Riza, though, that was mostly beside the point. No, what she eventually decided was that it was the way he talked, the way he spoke when it came to the future and what he planned on doing with his life when he was older, that captivated her so much.
While the specifics of his plan wouldn't come to light until much later in Roy and Riza's teenage years, Roy always talked about using flame alchemy to help others, to make things better anywhere it needed to happen. He spoke with finality about his plans, his tone never wavering and never expressing any doubts about himself, any second guesses. This was what he was going to do with his life - no ifs, ands, or buts about it. Coming from anyone else, it wouldn't have sounded nearly as final, as if he were the overseer of all things and knew for a fact that those things would happen. From Roy, however, that's exactly what it sounded like, and it was this surety he had about himself that Riza admired the most about him.
He would do it - she knew he would. He would have enough passion, enough firepower (both literally and figuratively) to make it happen. She believed in him from the bottom of her heart, and she swore to herself that she'd support him. In any way, small, large, or in-between, she'd show that, yes, I believe in you, Roy Mustang. I truly do.
"Will you follow me?"
So when he asked that question, it was easy for her to answer like she did.
After all, she'd been following him all along.