I've been wondering this for some time now (sorry if this has already been a thread). I have a few ideas on who it could be, but for the most part those just seem unlikely.

I don't think so on this one, because why would the letters still say "a friend" after you've met?

Nope, they want your power to grow slowly, with meditation and practice. They wouldn't send you out to get more words before you've even met them. (You can get courier messages from "a friend" before ever completing any of their story. As soon as you kill the dragon in the very beginning you can begin recieving courier messages)

I don't feel like it could reasonably be any of the main NPC's, because of the same reason - why would it still say "a friend" after you've met?
But in that case, who is it?

its m'aiq!

no, he's a liar

It's Major Nelson

Yeah, if it were ma'iq then you would just get to the cave and his whole caravan would be the only thing inside.

Its not delphine but I can see why one would think it would be.

Her leaving the note in place of Jergen windcallers horn.

being interested in you as the DB.

her being the supplier of the location of the Dragonstone.

However, you're met with suspicion by her initially.

Agrenir would also be a candidate, but he would likely call out to you rather than send notes, and you still receive notes even after meeting him.

I want to say you would probably meet our secret friend, but since they were not included initially I'm not sure what to say ;p

Nazeem

It's just that. A friend. Who? It's up to you to decide.

Of course, they may pull a Fallout NV on us and make it some important character that's connected to all the DLCs. Who knows?

If you're referring to the notes you get directing you to where you can find more word walls, then it's not that difficult, really. You get those letters after you use a shout in a populated area. Then a "friend" sends you a letter to help out. So you can assume that the mysterious friend is someone from the crowd that wants to remain anonymous.

As for why this friend remains anonymous, it's because the Radiant Story system isn't sophisticated enough to give that friend a name. You perform an action and that triggers an event. The best it does it keep track of what world space you were in when you triggered it, but doesn't seem to be able to keep track of who else was there with you.

Possible the friend is one the 9 divines (akatosh or most likely Talos) just trying to help the dovahkiin out.

I personally like to think that it's one of the characters we made from the previous games (i.e. the Nereverine or the Hero of Kvatch).

Just to throw a ridiculus option out why not say it may be an avatar of Talos? I mean, whomever sends these letters can sense you using the Thu'um anywhere so why not having it be the avatar of Talos? Seriously though, I have no idea who it is.

One player got one of these letters because he drawed a lot of attention at the Dark Brotherbood base. Soooooo....

Perhaps the Emperor. Not personally but through someone from the Penitus Oculatus...

Talos

In think its the OTHER DRAGON BORN!

The one you get from the DB isn't from your mysterious friend though, it's from the DB. It has their black hand mark on it and says "We know" or "We've been watching you" or something to that effect. I'm talking about the ones that direct you to a word wall that are from "A Friend."

Its the Kid from Helgen who now lives with Froki. Hamming.

The Guard's,most likely and they never give names and most are friendly to you?

Kynareth the creater of the voice

Oo I really like this idea. I roleplayed it was different people most of the time, who preferred to stay anonymous, but this is much better.

aske him and if he says no than we know it's him

I actually stoped getting the notes after my 4th or 5th character I got no friend any more

SO MANY SPOILERS!!

On topic,maybe but still most likely the guards

Yeah...I've never really entertained the idea that it was a guard or just some random person from Skyrim. The guards are too stupid, and the random people from Skyrim just aren't adventurous enough. (asides from people like Delphine, but it's pretty obviously not her)

I'm pretty much sure that it's one of the following:

1) A Divine
2) A player from one of the previous games
3) A Daedric prince
4) Talos (no I don't think he's a Divine...but no, I don't like the Thalmor)

But I got a friend from inside a nordic ruin once. That was full of drauger.

You should only stop getting them when you have all the words. I don't remember what it is, but there is generally a way to MAKE the courier appear. Look it up on wiki