Harmless Soldiers by 
thirty2flavors
What Regulus hates most about the Order is that they don't wear masks.
The Death Eaters live by anonymity. It is the crucial rule to every secret society. The Order does not seem to understand; they rush into the thick of battle with their wands at the ready and their hearts on their sleeves.
At first the faces are unfamiliar, and the names stir nothing in him. That's some Auror, there's another, there are those Prewett boys, anonymous face after anonymous face - easy to stun, easy to harm.
But it isn't long until he stumbles upon the rest. First there's Remus Lupin, slinging protective hexes in front of his friends whenever he gets the chance; then he sees the Mudblood with the long red hair dive to knock Peter Pettigrew out of the path of an Unforgivable; there's James Potter, still oozing confidence from every pore as he duels two at a time. All of them are a different story. People Regulus knows - hurting them requires a callousness he is nowhere near achieving.
Then, of course, there's Sirius, who runs into the fray, back-to-back with James as they fight off a three-hundred-and-sixty degree attack.
Regulus stands, transfixed, for a second too long.
Someone (one of us, he makes himself think) shoots a Killing Curse from across the room. It heads for the pair of them, but somehow (he always used to show off his reflexes with that bloody snitch) James shoves Sirius out of the way in time to throw himself to the ground. In the half-second before James moves again, Sirius' eyes are huge with horror.
Regulus spends the rest of the hostilities in a nervous anxiety that has nothing to do with self-preservation. He does not want to witness his brother's death, and he finds himself equally worried that James will die, or Lily or Peter or Remus; he is certain the loss of any of them would have the same effect on Sirius as Avada Kedavra.
It is hard, Regulus learns, to fight while aiming to miss, and when the end arrives and the Order has retreated, Regulus realizes the part of harmless soldier is one he won't be able to play much longer. He leans against the wall, breathing heavily, and runs a hand over his face.
On the other side of the room, Severus Snape stares at a blood stain on the wall, his face white. Regulus thinks of the redheaded girl who was brave enough to walk to the Slytherin table to talk with her friend, and he wonders if Severus was fighting the same benign battle.