Riza is not, nor has she ever been, in love.
Despite her quick climb up the ranks and a transfer to Central, her name is but a connection to the Hero of Ishval in Amestrian papers. And while those papers are not free to speculate, she hears the rumors and whispers over cups of coffee, related by equally weary veterans in need of an icebreaker with the Lieutenant that didn't broach their time in the desert.
Her motivations are skewered by the press and by her own comrades. They could never know the truth.
At times, Riza wonders if Mustang himself misunderstands her intentions.
Despite the Colonel's callous persona, Riza is the one who remains stoic—even when he cannot. She does care for Mustang's feelings, even encourages them with subtle glances or the tiniest of smiles at times. She thinks it good that he has such affection in the face of all he has done.
But she does not return it. And when Mustang scolds her instead of basking in the relief that she is alright, that she welcomed death and did not find it, Riza knows that he understands why.
She thinks back to her reasoning with the Rockbell girl. Far too difficult to justify gray areas and weave naïve fantasies to a child in Winry's position, Riza had found herself being honest. Though she loathes what her fellow soldiers would think of what she had said.
She does indeed have someone to protect. But it was, and still is, far from personal. Roy is the rising star of the military. He can reach the top with his talent and his leadership. Riza knows she is the key to unlocking all of his greatest potential, from the tattoo on her back to keeping him in line to allowing him to indulge his fondness, even when she could not bear to reciprocate it.
The nightmares are too disturbing, the phantom pain of her victims too manifest each time she enjoys herself. Too many children shot in the head to practice childish things. Too many men and women murdered in front of wives and husbands to allow herself the privilege she took from others. Too much faith shattered and dreams cast aside by pressing a trigger to believe her own happiness was possible.
Joining the military had been an application of ideals she hadn't really understood at all, and her ignorance had become part of a line of dominoes. The Homunculi pushed the first one, and all of the humans under them had not withstood the effect.
She had fallen in the desert, and risen again when she agreed to follow Mustang.
Riza is no sidekick. She does not care whether she is perceived as such or not, but she does care about what consequence her actions may have on her country. She is the last domino standing between the military's corruption and the one man who might be able to save it.
She protects Roy because he can protect everyone else. By protecting him, Riza protects everyone else. One mistake, one moment too close to him, and they could fall back into the childlike game of flirtation and utopian goals without vision. They would become blind, and Amestris would once again pay the price.
Riza cannot fall in love, and even if she could, she cannot afford it. She knows this after facing Lust, whom she didn't stand her ground against because for one minute too long, Roy became more important than Amestris.
She will not become lost in herself again.
Roy is by far the more powerful of them both, but when his heart or his flames falter, she must be the one to fire in the rain, against the rain, and against such attachment.
Roy can afford the luxury of those ties because she refuses to. It makes him a better man for it, better than the monsters he fights. But Riza may have to become a monster again to ensure that Amestris is not swallowed by one. And if Roy fails, Riza must remain, unwavering and unmoved by a tie that does not exist. She will not allow it.
Roy is the Hero of Ishval, but Riza is an iron pillar for Roy. Riza stands between he, Amestris, and the monsters, and one day she will reverse the domino effect she did not try to stop the first time.
It is the least she can do.