Interesting. It's not clarified what exactly happens to the more and child. It's possible she survived past the Catastrophe. But if she's experimenting to create her other half, where would she get the technology since most of it was destroyed/buried with Levianta?

This theory actually holds some water, as far as theories go.

I really like your theory, I hope at least some of it ends up being true. Nothing is implied that the 5th candidate's child was killed off, so it's very likely he/she's still alive.

As long as Levia isn't the singer of FML, I'm happy.

Who would she be creating in Full Moon Laboratory? It's implied that the person she's making is a mentor figure to her and a supporter of her experiments. Would it be Barisol? But it's suggested Barisol didn't raise her. Would it be Seth?

Could be Seth.

The time frame seems wrong, I admit, if FML is part of Project MA. 

The child born in the fifth project wouldn't be old enough to be a part of the experiments for further projects, unless I'm vastly misremembering the amount of time between projects. Given only about 14 years passes between Meta's death and the Levianta Catastrophe, unless this child grew to maturity abnormally fast. We don't see that with Hansel and Gretel, so it seems unlikely. 

IIRC, and what I had in mind while writing the theory (before I caved and made an account finally) was that the implications in FML were that the person being made was the person who had made the singer, not so much a mentor. With that thought, Levia and Behemo could be loosely considered the singer's creators, which would tie in with Project MA. If so, the singer of FML could definitely be hearing Levia and Behemo's voices telling her to continue the experiments - the person they're singing about "can't return" and so the project has to continue.

If, say, Project MA officially ended with the Levianta Catastrophe, perhaps FML takes place afterwards with fringe scientists who escaped the destruction and are still dedicated to finishing the experiment. Seth's ghoul children survived under Castle Hedgehog, as we know now, so it's possible that there were other researchers there still after the catastrophe.

That's more speculative than anything, but Castle Hedgehog just keeps popping up over and over again.

Since I was looking into it, I listened to the song a few more times - I think there's no way Barisol's Child isn't about the fifth project's result, but I'm not so sure that my original theory about said child being the singer of FML holds as much water based on something I noticed during these re-listens.

So I'm going to rework it a little.

One of the lines in FML refers to "combining the two into one" and noting that it doesn't mean that said "one" is worth twice as much. Sort of like a single baby that was supposed to be twins.

I think, probably, it's more likely that the singer of FML was part of the fifth project and the song is documenting this failed project's conception.

Maybe the singer is Marina Lucif herself - there's "Rin" in her name, she's linked to Project MA, and she's the ancestor to more Rin and Len characters.

Sounds like the kid in Barisol's Child is a boy based on the crossfade, so that kills the "singer of FML" theory unfortunately.

If anything, that's an interesting theory. Through I don't quite stick with the idea that if she was able to succesfully create her other half, it would be Irregular. I'm still sticking with the idea that Court!Irregular and Allen!Irregular were the unborn Cain and Abel.

It's funny actually, I actually have this theory that something like that happened, but it continued on to her becomming Waiter. She's the only Rin character that shows Len qualities (ponytail). It came from the fact that we found out that most of the awakened vessels of Sins aren't really what they appeared to be. With three being Masters, two unawakened, and the one for greed a posible holder of Adam's soul. This leave Waiter as the normal one? ...Yeah... But thats another theory entirely.

Anyways, there could be a chance, since both of them are looking for someone.

1. It's confirmed that Allen and Rillian are descendants of the children of Marina Lucif. The twins were taken away from her womb by the father, Vey Lucif, and he raised them on his own.

2. Waiter doesn't show us her supposed ponytail in any illustration.

I think the song isn't really about Levia and Behemo but their "human versions" or something. This theory may be far-fetched, but here it is: Levia and Behemo were - for some reason - re-born as humans, and so was the HER mask - Seth. "Levia" started to study human's "hearth" and "Behemo" wore women's clothes. It was the last years of Second period, and Malice started to destroy the world by causing people to have HER. One of the unlucky people was "Levia" who started to have "unending murder impulses". She asked "Seth" to help her. He told "Levia" to kill her "other self" which means the real Levia. At the same time Levia saw "Behemo" after killing his wife (=the doll whose "limbs were warm and in pieces") and tried to talk to him trough the mirror.

Soon after Gods started their escape from Second world, of course with Levia and Behemo. "Levia" tried to kill Levia, but before she did it she realized that this was a trap. If she killed the real Levia, there would be space for Seth to escape with the gods. (But wait - Seth did sneak into the Ark? Well... I really don't know about that ^^;)

This is what I think. Now I just have to wait for you to tell me 101 reasons why this theory doesn't make sense xD

Well, since you brought it up:

1. The "other self" that Seth was referring to wasn't "the real Levia", it was Behemo.

2. Levia and Behemo are not human, at least not human as-per the humans in Evillious. As such they could not have been "reborn" as humans, and even if they could that's not how rebirth works in the series (it certainly wouldn't create two Levias or two Behemos).

3. There's nothing saying Behemo married his sweetheart, and him killing her is pure speculation.

4. Levia did not pull Behemo through the mirror (if that is indeed what happened as the song is vague) before attempting to kill him. She was planning to kill him, changed her mind upon realizing the trap, and then took him with her.

I'm sorry but your theory doesn't really fit with pre-existing information on who Levia and Behemo are or with the events that are described by the song. There's no reason to think it's about different versions of Levia and Behemo rather than just being the gods themselves.

Although there are several theories to be made about Seth, but then we're not even sure that's him in the song.