[ Chapter 2-3-4-5 Spoilers/Theory ] Who exactly is DEVICE_FRIEND?

Intro:
So, I played your Chapter 5, even fully cleared it content-wise, and what can I say: thanks Toby for forgetting to add the lore bomb and then dropping it in a patch instead, real generous of you. Getting to the point: today we're discussing who DEVICE_FRIEND is. So yeah, speculation, let's go!
The core of the theory:
Personally, I believe DEVICE_FRIEND is a program. More precisely: a daemon {Daemon - a program that runs in the background without user involvement} whose job is to deal with problems in the code. I also think MARE (a.k.a. Nightmare, ERAM - I'll just call him MARE in this text) is someone who can use it. More on that later, in the MARE section.
Also, saying this upfront: I believe the eggs are something foreign to Deltarune's world, which is why DEVICE_FRIEND hunts them. And the man behind the tree is Gaster (speculation). I'll cover this in the eggs section and partly in the Weird Route section.
Part 1, MARE:
Getting into it - let me remind you that from PINK's monologue in Chapter 5, which was added very recently, we now know there was someone who liked cats so much that they owned an entire hunting pack of them. In my opinion (again, this is speculation, blah blah blah) that someone is our MARE.
You could immediately ask why, and I have two points here. First and most obvious: the fact that he literally summons DEVICE_FRIEND in crowds, which most likely means he isn't the only one, there are in fact many of them. And second: he's very much in favor of us following the "Weird" route, which I'll cover in that section (short version: on the Weird Route we physically cannot encounter DEVICE_FRIEND).
Part 2, DEVICE_FRIEND:
Now the good stuff! Talking about DEVICE_FRIEND, I'll throw down my trump card right away: here's what he's called in Chapter 5's files : "gml_Object_obj_plat_testtarget_Create_0" - literally "test target."
(Correction, thanks to u/sailing94: I originally wrote that the Chapter 5 encounter requires the Egg. That's wrong. The Egg and the DEVICE_FRIEND encounter are two separate triggers, completing both, in any order, is what unlocks Pink's shop dialogue. Several guides state this incorrectly and I took their word for it. My bad.)
I'll also say upfront that DEVICE_FRIEND is hostile toward the eggs. No, that's not even right... he hunts them, because they're glitched pieces of code (or something foreign to Deltarune's world, that's where we have room for speculation).
Part 3, The Weird and Normal Routes:
In this section I'll cover both routes, starting with the "Weird" one. On the Weird Route, eggs are only available in Chapter 1 (because we haven't started the Weird Route yet), Chapter 3, and Chapter 4.
Precisely because of this - since we can't collect all the eggs on the Weird Route, and Chapter 5 tells us outright that we're skipping Chapter 6 - I have a theory that we'll encounter FRIEND in Chapter 6 if we collect all the eggs, and won't be able to encounter him on the "aborted" Weird Route.
I'll also point out that on the Weird Route, DEVICE_FRIEND cannot be legitimately encountered (relatively speaking: technically he's in Ralsei's cutscene, but he doesn't appear directly there, and the MARE fight plus the minigame in Mike's room are Sword Route, a separate path, so it's 50/50). My theory is that since everything is following the prophecy, the "correct" path - he has no reason to intervene. Or it's simply caused by the practical absence of anomalies (eggs) on that route - the question remains open.
As for the normal route: we can encounter DEVICE_FRIEND 6 times. First: in the Queen's basement, in one of the rooms he appears BEHIND the room for a split second (I think he was waiting for Spamton, more on that later in section 4.5). Second: in the cutscene with Ralsei where he explains how the Dark Worlds work. Third: in the MARE fight, where he summons "friendos" (that's just what I call them for fun) in crowds. Fourth: in the dark room in Chapter 4, where it's practically impossible to encounter him on a normal playthrough unless you're specifically looking. Fifth: in the minigame in Mike's room. And sixth - in Chapter 5: again, he can only bite us if we've found the Egg.
You can probably already see where I'm going, but I'll say it directly: my theory that the Weird Route is the "correct" one from the prophecy's perspective comes from the fact that DEVICE_FRIEND constantly watches us on the normal route. I think this is because on the normal route (just a reminder that one of Ralsei's lines says he thought that if we were kind enough, he could change the prophecy - which most likely means the original prophecy is pretty brutal) we are, first of all, "pacifists" who only fight when absolutely necessary, and second: Susie might not even be the second hero (the theory that Susie is the Dragon and Noelle was supposed to be the second hero fits perfectly here, but that's speculation).
I'll also remind you that MARE and the whole Sword Route basically hint at the existence of the Weird Route. Done with the routes, on to the second-to-last section!
Part 4, The Eggs:
Alright, let's go. There's a lot to say here, but let's start with the basics. On the normal route there's one egg in each of the 5 chapters (on the Weird Route there are 3). Also, a reminder that eggs stay eggs regardless of which world you're in, and the man behind the tree is the one who gives them to you.
I have every reason to believe the eggs are something foreign to Deltarune's world. If that's true, then the reason DEVICE_FRIEND hunts the "protein" (literally the eggs, ahem) is that they don't belong to Deltarune's world. Literally.
Most likely, the man behind the tree (or Gaster) won't give us eggs in the future - I'm guessing because DEVICE_FRIEND has ALREADY taken notice of him, just like he has of us (the achievement for completing a chapter "without issue" is withheld ONLY if you collect the egg).
On the Weird Route we can't get all the eggs at all. This is most likely because on the Weird Route we SKIP Chapter 6 and jump straight to Chapter 7. Under that scenario, if we encounter DEVICE_FRIEND as a secret boss in Chapter 6 and can only fight him with all the eggs collected, it makes sense that on the Weird Route we don't need all of them.
BUUUT this raises a serious question: why would man behind the tree (or Gaster) give us the eggs at all? Annnd I don't know. I mean, I could guess that they're pieces of Gaster and all that, but it's too early to say anything.
Part 4.5, Spamton (this section is entirely hypothetical):
This section is basically a continuation of the last one. I think Gaster's first contact wasn't even with us - it was with Spamton. Someone called him and made a deal with him, after which he became wildly successful, right? What if that was Gaster?
They made some kind of deal where Gaster helped him become successful, rich, and so on, but asked for something in return. But before Spamton could fulfill his end of the deal, DEVICE_FRIEND found him. Gaster realized what was happening and cut off all contact entirely - leaving Spamton alone, with DEVICE_FRIEND on his tail.
Conclusion:
Well, this theory is built on speculation layered on top of speculation... but that's what a theory is. I just want to add my own take: I don't consider DEVICE_FRIEND a villain in the usual sense. To me it's a function: a function responsible for one specific job, namely making sure Deltarune's world (which, as we've long known, is a game) doesn't collapse from things like the eggs.
It'll be interesting to see whether this theory stands the test of time or dies like the old one. We'll see!