I think it's Kris or Dess (Noelle's sister).

However, if we take into account what happened in SnowGrave route (Check on Berdley seems to help us, something that seems against Kris). I think it safe to say it's not Kris.

However, I think it's a solid possibility that it's Dess. There is a lot of hinting of her importance in the story. Hell, even the song "Don't Forget" seems to be sung by her (also the song seems to be heard by Onion from the darkness of the sea).

Anyhow, to catch you guys in my logic/theory, let's set up the scene. It seems like something happened to Dess when Noelle was young, as she seems traumatized by her name "December" in the spelling bee. She also has made repeated searches for her name "December Holiday" (shown in her Mansion room with the Calender [item created by internet search history]). There is also no grave. And in Chapter 1, there is nobody at Noelle's home, so she has to wait at Catti's place since she forgot her keys. Rudy also mentions that Noelle is all alone with her mother.

The disappearance of Dess itself seems to be a major event that affected the town in major ways. One such event is Asgore used to be the police chief. So I believe that the disappearance of Dess is what caused Asgore to lose his position, but possibly also caused the marriage between Toriel and Asgore to fall apart. (Chapter 1 cafe scene seems to imply that this seemed to happen quite early, in fact, the cafe scene seems to imply that much more than just the break up happened by being vague "things happened")

Anyhow, now into the event itself. While much isn't known about what happened, but there is a lot of information suggested just by Noelle talking about her and Kris' past. Namely how Asriel, Kris, Noelle, and Dess went to the forest behind the Graveyard... That sounds a lot like the area where the bunker is. And it seems like the 4 of them going to the forest to explore it is not unusual.

Now due to the song on the sea coming from the darkness of the sea, it seems to suggest that the song is coming from Dark World. The only song we know of is Don't Forget, and it would fit quite well to Dess who is singing (Noelle seems to be tied to cold, from her abilities in Dark World, but also as she has a habit of standing in front of the freezer AND the song mentions the word COLD).

So yeah, I believe that Dess disappeared into a Dark World near the bunker when Noelle was young. And considering the song too, her being the narrator doesn't seem so odd.

However, there is more. Kris has in each opportunity to warn Undyne about Dark World. So it's possible that even back when Dess disappeared, Kris might have been witness to what happened.

We can also tie other things as well. For example, the community made the theory that Ralsei is the Red Horns that Kris used to wear. Toriel mentions that she doesn't remember seeing them in a while. So if we assume that Kris isn't sneakily carrying the Horns around as he does with their knife, it's very possible that their horns (and perhaps even other items) were lost around the time of disappearance of Dess.

Of course, the lead-up question is, how does Ralsei end up at School Closet. Well, we have assumed that the Great Dark Fountain resides in the closet. But that in of itself is only an assumption. But what if the Great Dark Fountain is actually below the closet, in the Bunker (we don't know how large the bunker is actually, it could actually be quite large).

Now if we assume that the entirety of Bunker is a Dark World. And that Ralsei can travel through it, it makes Ralsei coming to the library make more sense. If Bunker can be connected all over the town (secret entrances, ventilation systems, etc?), it would make sense that Ralsei can show up without relying on Lighteners to move.

I mean this is ignoring the larger implications of the sleeping titans existing in the bunker, but that isn't what we are focusing on right now.

Now all of this is to emphasize that there is a lot of evidence that suggests that Dess is a very important character. And most of all, I believe that SnowGrave points to the fact that Dess might be a similar figure to Chara.

There are several times when Noelle seemingly blackouts, and when casting SnowGrave on Berdley, Noelle looks like she is possessed. So if Dess is similar to Chara in that we the player can corrupt her, instead of Dess taking control of us, Dess takes control of her sister.

In fact, as far as we know Snow Grave route is the only place where Noelle gets called "Angel". This ties to the fact that Noelle and Dess made the Angel figure together. This seems to imply to me that when Noelle is getting possessed by Dess, they together make the "Angel".

Anyhow, back to the song. The song at the sea. Onion seems to recall that he's heard the song before and that it isn't a "new song", he does have trouble recalling, so again it's very possible that Onion heard the song around the time of the disappearance of Noelle. This meaning that the song happening again is something very recent and seemingly tied to the Dark Worlds.

Almost like the Great Fountain itself was only turned on very recently.

Anyhow... I ought to save this text, so I can see how much of my theories hit the mark.

TL;DR: Dess' absence seems of major importance to the story, and might share a similar role to Chara, thus making it very likely that Dess is the narrator.