Spirit is sixteen years old when he finds the first scar.
More accurately, Spirit is sixteen years old when he notices the most recent scar, which is fresh. In his subsequent panic he discovers exactly twelve others, and there's only one person that could have made them because there's only one person, one backwards freak that makes stitches like this.
He's in the shower. He screams in frustration, partly because he's covered in scars from where his partner cut him open and partly because he hates himself because there are twelve of them and some of them are old enough to be fading already and he hasn't noticed until just now. He's going to go to Lord Death, right away, he's going to demand a new partner. Sorry, boss, he practices the upcoming conversation in his head, but I can't babysit this fucked up little shit anymore. Find somebody else who's stupid enough to hold the leash on your favorite little attack dog.
But as he finally gets his breathing back under control, he slowly realizes that he can't do that. Who else would put up with Stein? None of the other DWMA students will even look him in the eye. None of the foot soldiers in Death's Army will look at him period, when Stein and Spirit run missions with them, and that's been happening more and more often lately as the war heats up. And Spirit knows their involvement is only going to increase because for once the witches aren't backing down or retreating back into secrecy and they're gaining allies, and because whether the majority of the student body wants to admit it or not Stein is the strongest meister to ever pass through the DWMA's doors and Lord Death would rather kill him than risk the other side being able to use him. Spirit's not stupid, just a little slow, and he can sense that in this scenario, he'll be the one punished for abandoning ship. So he swallows his bile and climbs out of the shower. Gets dressed. And when Stein emerges from his bedroom, still looking half-asleep, Spirit tackles him.
Stein is fourteen when his weapon partner loses his mind.
Or, more accurately, Stein is fourteen when his weapon partner figures out that he's the subject of the grand experiment Stein has been vaguely going on about for the past few years. And while Stein, at fourteen, is extraordinarily strong for his age, Spirit has two years on him and is a weapon, which explains how Stein finds himself pinned on his back several seconds after Spirit tackles him on a Saturday afternoon.
Stein remains relaxed as Spirit aggressively rummages through his pockets, but unease spikes in his chest when his partner triumphantly pulls out the scalpel that Stein keeps on his person at all times.
"Spirit..." he says slowly, not comprehending, even as Spirit pushes his shirt up to his neck.
"Shut up," advises Spirit, who is now practically sitting on top of his meister. "I don't want to hear it. One for every one you gave me." Before Stein can protest Spirit makes the first cut, right under Stein's ribs. It's jagged and not nearly as neat as anything Stein has ever put on him, but that's not the point, the point is making sure Stein never touches him with a scalpel again and he forces himself to keep going as Stein starts to fight him, kicking and screaming as he makes the next cut on the opposite side.
By this point in his life Stein has figured out that he has a fear of being, for lack of a better term, dissected. The things he does to other living things, he knows he could not handle being done to himself. He knows this, but until this exact moment in time, pinned on the floor of his apartment by his partner, he did not know how much worse that fear was when the one person he trusted was the one doing the cutting.
"No," he screams, voice hoarse from effort, "no, no, no, no, please, no, I'm sorry, Spirit-"
But Spirit is determined, and by the time he makes incision number twelve Stein has given up and is limp underneath him, crying. He doesn't move when Spirit finally clambers off of him, either, and though he twitches brokenly when Spirit returns with the materials required to stitch shut twelve shallow cuts, he doesn't fight. He thinks he's done fighting because he wants very badly for Spirit to stop punishing him and he can't think of any other way to make that happen.
Spirit's stitching is clumsy at best, and Stein's scars are going to last as long, if not longer, than his will.
"No more fucking experiments," he warns hoarsely, and leaves Stein trembling on the floor to go the bathroom and wash up. He washes his hands first, then leaves the water running while he throws up in the toilet, then rinses his mouth out and washes his face in an attempt to look cool when he exists the bathroom. In their living room, Stein is still on the floor, chest and stomach still exposed along with Spirit's bad stitch job.
"You're such a crybaby," Spirit says with forced apathy, and that is the end of that. And the beginning of a future that is far different than it could have been.