I already suspected that Asriel was the unused speaker before Chaper 5 came out, but now I believe that this is more likely than not.
Argument 1: The monologues happen in the present time
I am certain that the speaker is talking during the chapters, not in the past. There would be no in-universe reason for the monologues to be chapter-specific otherwise.
In Chapter 1, the speaker is reacting to being recently trapped.
Where...
Where am I?
Hello...? Anyone...?
Is... is anybody out there...?
Someone!? Anyone!? Can anyone hear me!?
...
It's dark.
It's so dark here.
Someone, anyone, if you can hear me...
Say something... please...
And in Chapter 4, they estimate the amount of time they spent trapped.
I'm starting to lose track of the time.
Has it been days? Hours?
If someone told me it's been years, I might believe them...
While the speaker entertains the idea that they might have been trapped for a ridiculously long time, they are a lot more confident in their days/hours estimate. Two days have passed from the start of Chapter 1 to the start of Chapter 4, so the math checks out.
December has been missing for years before Chapter 1, so this can't be her right after she went missing. Meanwhile, nobody can reach Asriel after the Internet went down, so he could have gone missing right before Chapter 1 and no one in Hometown would notice. (More on this later.)
Argument 2: The speaker had nightmares about the day when December disappeared
I...
I always did have that nightmare.
Walking into the darkness,
With the light shining from the doorway...
Then the door slams behind me.
And everything goes black.
The speaker is describing a recurring nightmare they had while still in the Light World. The nightmare is exactly like what happens to Kris and Susie at the start of Chapter 1, so we can assume that the speaker went through a traumatic event involving a Dark World long before they got trapped in the dark.
For the sake of simplicity, I believe this is the same event that traumatized Kris and Noelle, and also the “horrible day” Asgore mentions in Chapter 5. One singular inciting incident haunting the narrative. This means that the speaker can't be December. She wouldn't have time to get recurring nightmares after that day, because she would have been gone already.
Argument 3: You forgot about breakfast
Before, the sun came up,
And so many wonderful things happened that made time meaningful.
Things like breakfast, or late night TV...
The “late night TV” part refers to the tradition of the Dreemurrs and the Holidays gathering in the Dreemurr residence to watch TV, so it equally applies to both Asriel and December. The “breakfast” part, however, applies to Asriel a lot more. Chapter 4, the same chapter when this unused monologue happens, establishes that breakfasts in the Dreemurr residence are awesome.
Kris, how the hell'd you sleep through breakfast like THIS!?
Butterscotch pancakes? INFINITE omelettes?
Living like this's gotta be a sin.
Argument 4: The unused speaker is written in a gender neutral way
Disclaimer: I don't speak Japanese.
... But people who do claim that the Japanese localization of the UNUSED monologue is written in a way as to hide the gender of the speaker, which is a deliberate choice in Japanese as opposed to English. If the speaker is really December, and we're supposed to have it figured that out already, there would be no reason to do that. This suggests that the speaker isn't actually a girl as we suspect.
Argument 5: What the heck happened to Asriel?
After Chapter 5's Side B, I am entirely convinced that nothing exists outside of Hometown.
It wasn't always that way. In the past, people could go in and out as they pleased, and connect to the rest of the world through the Internet. But the world that we experience since Chapter 1 is limited to Hometown, which is why there is no Internet.
Asriel was deemed important enough to carry over to our version of the Deltarune world. But since he wasn't in Hometown at the start of the game, the program didn't know where to place him. This is why he is unused.
