"Hello, Everett!" Clef called out cheerfully, as he strode through the door into Dr. Mann's office. He immediately hit the floor as a hail of bullets passed through the area he'd just been standing in. He crawled on his belly towards the good doctor's desk, even as Mann continued firing wildly, yelling the entire time.
"I DON'T CARE WHAT SCP-001 IS! I DON'T WANT TO KNOW WHAT THE ORIGIN OF THE FOUNDATION IS! WOULD YOU PEOPLE JUST LEAVE ME ALONE!" Mann, a little unhinged, continued firing until the clip went dry, and then continued pulling the trigger, just on the off chance he might have missed a bullet. Clef reached up and, carefully, took the gun from Mann's hand.
"Having a bit of trouble, Doctor?" Clef asked, as he subtly replaced the gun with a flask of home brew. He checked the gun absently, then tossed it into a corner.
Mann chugged on the flask, not caring that it burned, or that the taste of apples was absolutely overwhelming. It was brain death, and it was a welcome relief from the near constant stream of Overseers baring their soul. "They…they won't stop Clef. They keep coming to me, and unburdening. I'm not sure how much I can take. Twelve hours from now, O5-13 comes in to tell me his story. He left ten minutes ago. I can't stand that guy!"
"Relax, Everett, relax. You're with a friend now. Come on, take a seat, take some more drink…" Clef casually closed the door, and locked it. "You know, as long as you're hearing the secrets, what's one more, hmm?" Clef nodded, picking up a chair and setting it so he could kick his feet up on Mann's desk. "As long as you're listening, why don't I tell you where I came from?"
As Mann whimpered and sought solace in the flask, Clef began to speak.

It's so long ago, I can barely remember it myself. I wasn't always a field agent, you know. I used to be a researcher, specializing in humanoid SCPs. It was horrible, Everett, back in the beginning. Humanoid skips got the short end of the stick. I'd begun lobbying for some changes. Trying to help people, just a little bit. So they demoted me. Assigned me to some of the anomalous skips, that we didn't know what they were. So there I am, trying to figure these things out… and, coincidentally, someone fucks up on containment of 76, and next thing I know, Able is bearing down on me with a bloody meat cleaver.
He got me. He got me bad. I shut down, to avoid the pain. Even now, all I can see are flashes of light along the blade… The Foundation fucked up. They thought I was dead. Tossed me in a mass grave, and forgot about me.
But I was alive. I crawled out of a grave filled with D-class. Rotting bodies all around me, and I still managed to crawl out from under all that dirt. Make my way down the road. I killed a man, just for his clothes and vehicle. But I didn't think about it, at the time. Thought I was just, y'know, tough.
I made my way to a hideout my brother and I had set up years before. One of those 'just in case' type of things. I spent a month there, treating my wounds, recovering my strength. And realizing, I was free. I had a mind full of Foundation secrets, and the Foundation thought I was dead.
First thought was MC&D. I could sell all I knew, make myself a billionaire, and never have to worry again. But… I couldn't stop thinking about the humanoid skips in Foundation custody. And about Able. We could have done something about him, could have ended him. But no, we had to contain… I actually even thought about going to the old man, but you know how he gets. So I went to the GOC.
They were skeptical. Who wouldn't be? But, in exchange for some plastic surgery, a whole new identity, I gave them everything. They made me an agent. I made sure I was their top agent, Ukelele. And I chose my new name, Alto Clef.
But it was when dealing with a young lady whose only problem was that she was part goat, that I found out I was immortal. My associate wanted her dead. She was a Green, albeit a weak one. We argued. He shot me, and then her. I didn't die. I shot him back, and held her while she died. That girl… she's stayed with me, Everett.
I digress. The GOC wasn't for me. So, I made overtures to the Foundation. If I couldn't defeat them, maybe I could subvert them from within. No one was more surprised than myself when they sent me to talk with me. But it made my cover complete. None of them have ever suspected me. My tentacles stretch throughout this Foundation. I can take it down whenever I need to.
Just thought you should know.

Clef pried the flask from Mann's unresisting hand. Over the course of the story, the doctor had made his way under his desk, where he now just sat and rocked, whimpering to himself. The Agent smirked, and began to exit, when Mann managed to croak out a response.
"Alto…Why?"
Clef just smirked. "Please, Everett. We've known each other for long enough. Call me Jack."