It's a simple thing, in the end, cool leather with a little buckle at the back. No decoration, no coloring, no names, nothing except the quiet weight of anonymity. A meaningless object for a meaningless object, and Leo's belly grows warm at the idea of being so easily effaced. 
He holds still as Raph approaches him, holds still as leather slides into place, holds still it clicks shut behind his head. He won't take it on or off, ever--that'll be Raph's responsibility. There are no keys or fancy locks, just the simple trust that Leo will wear this for as long as his brother needs him to. 
Raph runs his fingers around Leo's throat, adjusting the collar, but just as clearly savoring the way it presses against Leo's skin. His breathing isn't constricted, but there will be a mark when it's eventually taken off, a reminder to follow him into the waking world. 
He rests two fingers on the pulse of Leo's throat. "You good?" 
Leo nods stiffly, breath picking up a little. He'd underestimated how quickly he would react to the presence of something so simple, wanting creeping through his veins to stir his loins. Judging by the faint change in Raph's scent, the way his free hand curves around Leo's hip, it's hitting him, too. 
Raph's steady, deadly fingers wrap around the collar and tug; Leo follows easily, turning his head to meet his brother's kiss. Raph kisses as forcefully as he ever does, but there's something almost triumphant in the way his lips curve, like a battle's been won, even if he's had Leo for a while now and they both know it. 
They kiss again and again, Raph driving him back towards the bed and toppling him down under it. His hand slips down and Leo automatically lifts his tail aside, exposing his carefully cleaned, delicately opened, hungry little hole. 
Raph yanks it out and pushes inside without hesitation, a conqueror claiming his prize. Leo lets a little churr slip free, arching up into the hand his brother still has wrapped around his collar, legs wrapping around his brother's waist. 
"You're mine, now," Raph whispers, his voice a low growl in Leo's earslit. "Understand? If you try to run away, I'll find you, and I'll drag you back home by the ankles if I haveta." He yanks on the collar again, pulling Leo into another bruising kiss, whispering into his mouth, "You ain't getting away from me, ever." 
And he knows it's all just play, but the words make Leo's head light anyway, warming him almost as much as Raph's steady, relentless cock. He clings to Raph and whimpers, overwhelmed at the thought of being owned like he hasn't been since...since... 
He stiffens, ever so slightly. 
"Hey," Raph barks, giving the collar another tug. Leo looks up at him, biting his lip. "I said mine. Not. His.” And then, gentler, “Remember the safeword.”
Leo looks into his eyes and takes a deep, slow breath. “Macaroni.” It’s simple, easy, inoffensive.
“That’s right, babe.” It’s Raph’s voice ecohing in his ears, Raph surrounding him and filling him up, Raph stripping him down to an object, a toy, and toys don't have to remember bad things, or what it was like to be used by the wrong hands. Toys just have to be.
"Not his," Raph whispers again, yanking him close. Leo holds on with both hands, eyes closed, pulse humming against the collar--the collar, Saki never used a collar, Saki never owned him like this. 
Raph runs a thumb along the leather, keeping him close, keeping him grounded. Leo relaxes into his brother's touch, tension flowing away, and believes.