He is an enigma to to me. Well, most people are, Grace - yourself excepted. Look at the two of us, Mr Reese and I. What an odd couple we make. Comically odd: sometimes I feel like the goat or donkey which some sleek, powerful racehorse inexplicably allows to share its stall. What I do for Mr Reese seems obvious enough at first glance. I took a homeless bum bent on self destruction and gave him a job and a purpose. What he does for me is less evident.

When I started this whole thing I looked for a partner with the skills to intervene, and naively I thought that meant the Darknet. First there was Mr Symes. He worked three numbers, of whom we only saved one. Then he told me it wasn't his "thing", whatever that is supposed to mean, and accepted a rather large payment for his silence. I still keep an eye on him - you can't be too careful. The last time I checked he was in West Africa involved in something to do with diamonds which doubtless wouldn't bear close scrutiny.

Then there was Mr Bolton. He only worked the one number, successfully as it happens, before he simply disappeared. When I looked I found him working for the Zambrano organization. I've no idea whether he survived Elias' takeover, but it doesn't matter. He knew almost nothing about me, and I never let him near the Library.

And then of course there was Mr Dillinger. What a fiasco that was. Five numbers, six if you count Daniel Casey, nearly all resolved satisfactorily. But once I had recovered from the aftermath of his betrayal I realized that all three of my partners had had a fatal flaw. They weren't broken enough.

You see, Grace, I needed someone who was dangerous enough to be an avenging fury or a heroic rescuer, but dependent enough upon me that he wouldn't betray me. A tool sharp enough to use, but not so sharp it would cut me. Does that seem cold? Cynical? You always saw the good side of me, Grace. In someways I'm glad we were parted before you saw the darker part of me, the man who lies and manipulates with the ease of long practice. In any case, no more reaching out to people on the Darknet.

After the Daniel Casey situation, Mr Reese was in the back of my mind. I had certainly been aware of him as a possible threat, but he surprised me when he let Casey go. It didn't really occur to me at that stage that he might be the answer to my problem, and in any case he and his partner disappeared completely straight afterwards.

Those months after I let the laptop go were... difficult. No partner meant there was little I could do, half-crippled as I was, to help the Numbers. And I was aware of that laptop, out there tracing a trajectory like a missile. Where would it land? But it was in New Rochelle that everything suddenly clicked into focus.

I was in the hospital there, mixing business with, well, not pleasure – physical therapy and more surgery on my abused neck. The business side was a painful conundrum involving an old Number, Jessica Arndt. She'd been murdered by her husband – nothing I could do to stop it stuck in a wheelchair. But then the husband's number had come up. That was odd enough to lure me out of the city and cause me to opt to have my latest round of surgery at New Rochelle. If I hadn't been able to save the wife, maybe I could save the husband. Or stop him from killing again. But once I was there it became obvious that there was a triangle involved. And the other vertex of the triangle was Mr Reese. I was there when he arrived to try to make contact with Jessica. And I saw the look on his face when he was told she had died. The expression of someone whose entire world has just been swept away in a millisecond. When he walked out I knew Peter Arndt's fate was sealed. And again, there was no way to stop it.

When I got back to the city I dug in to what had happened to cause Mr Reese to appear at that hospital, injured and bearded and evidently no longer working for the CIA. I was horrified to find that another bystander – though I wouldn't call him innocent – had been caught up in my ever-expanding sphere of catastrophe. The laptop. What warped, malevolent deity had guided my attempt to make something better to such a destructive conclusion?

But suddenly the whole awful, twisted situation seemed to make a kind of sense. Mr Reese felt responsible for Jessica's fate. I felt responsible for Mr Reese, having sent out the laptop. There was that look on his face, too. I knew where that came from. That moment when a brutal universe suddenly smashes all your hopes and leaves you utterly bereft. And he certainly had the skills I needed. Was there some way we could help each other?

But finding him again was difficult. When Peter Arndt disappeared, so did Mr Reese. I built an app to alert me when any of his aliases showed up, but for month after month all I got were false alarms. For a while I feared he'd killed himself, but then imagine my surprise when he surfaced in a New York police precinct. Yes, yes, he rebuffed my initial attempt to get him to work for me, but I was confident if I could just pitch it to him right he would help me. Because I could see a difference in him. After all, he'd let Casey go.

And so there we were. We were both cripples in our different ways. Maybe between us, though, we could just about add up to a whole person. And one way or another, we were going to make a difference. But something strange happened after a while. I wanted someone who would be dependent on me, so I could be safe from them. But I never anticipated the way I would come to lean on Mr Reese.

Years ago I came across a rather harrowing account of a pair of Siamese twins kept as a curiosity by an English earl several centuries ago. When one of them died the other lived another five days, dragging his brother's corpse along with him and "moaning most piteously". I know it must sound morbid, but I wonder now how long I would outlive John, assuming of course that the matter isn't otherwise taken care of.

The Numbers never stop coming, and each and every one represents another chance. Another chance to save something, to make something better. Sometimes we succeed. Sometimes we fail. The failures are bitter, the victories transitory. But still we keep at it. There's no end in sight, and I find it hard to pinpoint exactly what it is that propels us into each new case. My head tells me that there's no hope of ever really making a difference, that we're like the figure in the Greek myth, endlessly pushing a boulder up a hill, only to have it roll back down to its starting place again. And yet every day, there we are. Putting our shoulders to that great rock, pushing body and mind to their limits to try to save someone. One more time. Together.

All I can say is that if you tunneled down, down past the employment, past the friendship and the trust, down past shared danger or congruent values or even some subconscious sexual attraction - if you got all the way down to the naked bedrock of the soul, you would find the same stuff in each of us.

Well. It's very late now, and our new Number will be emerging from his crap game into the sights of the hit man after him. I need to be monitoring Mr Reese as he goes about his part of our business. Wherever you are, Grace, and whatever you're doing, I hope you are well, and happy, and all things bright and joyful. Good night.

POI*POI*POI*POI

He's a mystery to me. You know, it's funny, Carter. I sit here on a stakeout and think of all the conversations we've had sitting in a car late at night. You were always easy to talk to. Harold, less so. I'll never forget that shocked silence the time I explained why he needed an empty water bottle on a stakeout. Though the trick with the potato chip can was pretty good.

But yeah, Harold the really private person. Even though I know a lot more now, he's still a mystery.

The first time I saw him, he was this funny little man standing there with his big soft "security" guys. And he tells me this whole line about knowing when bad things are going to happen. I wrote him off immediately as a nut job. Besides, my blood alcohol level was dropping and I needed to find a drink before the DTs set in. But the really weird thing, Carter, was that he was the second person in hours who had offered to help me. Funny, huh? Months, years go past in which you're all alone, and no-one's coming to help you. And then all of a sudden, people are practically throwing themselves at me. Once I had enough bourbon in me to stop the trembling, that seemed pretty funny. Hilarious, even. Though I wasn't laughing.

So he finally persuaded me to get on board with him. Said he'd never lie to me. Actually I'm not so sure he's kept to the letter of that promise, but it's true. He's never deceived me about anything really important. And even then, he's only done it to protect me. Which is pretty strange when I think about it. He protects me?

Another funny thing, Carter. Remember you said trust was the basis of any healthy relationship? It didn't come easy for Harold. Or me. When I started out trying to figure him out I did all the usual things – tailing him, observing, trying to burrow past his defenses. Fusco ever tell you there was a time I had him following Finch? But in the end I realized I didn't need to know. Yeah, I was curious. But I wasn't going to push any more. Because in the end Harold's actions were what mattered. And yes, he'd made mistakes. Paid for 'em too. But what was inside was… hard to find the word… decent. Kind, even.

I try to put into words what Harold does for me, but I'm not really a words kind of guy. (And I can see your eyes rolling and hear you say, "you think?") He said I needed a purpose, and gave me one. He gave me someone to trust when I knew I could never trust anyone, ever again. He's saved my life a couple of times, and I've returned the favor. We're a team. Almost, a family.

Ah. Here comes our Number – and the mob hit man who's after him. Showtime, Carter. Wherever you are now – Godspeed.