IMPORTANT: unless you are at least moderately invested in the binding of isaac's themes, symbolisms, and "plotline", this will probably mean nothing to you. carry on if this is not the case.

The Siren, design wise, is both shockingly well-fitted within the more vanilla and Edmund-designed content of TBOI as well as providing something new to fight... or even to ponder.

Character design wise, she actually lines up very well with pretty much any other demon character in the game. The dark grey skin and weird eyes at the very least -- yeah, that sure is a demon. However, her key design point, what sets her apart from something like The Dark One or the two Horns, is the aspect of femininity. This doesn't require giving her boobs or anything drastic, either; it's mostly in the hair and the voice. It is important, actually, that these are the two most notable.

VERY notable, in fact, because one of the key themes of TBOI is the mixture of specifically motherhood and violence, seeing... well, it's Isaac. It's the game where your mom wants to kill you. It's the game where it centers on the THOUGHT that your mom wants to kill you, and also that Isaac, over all, if it meant ending that constant fear of his mother, the only major figure in his life, would and did, in fact, kill himself. Isaac's Mom is a constant presence within the game, of course. Even after the mandatory per-run defeat of her, you end up within where you were born, and you also sometimes end up still haunted by those annoying big hands with the cross-rock thing oh my god annoying. Everything in this game is meant to be a reflection of the mental state of Isaac as a person; every single little thing possibly means something deeper if you take the time to analyze it. Probably. Okay maybe not everything I don't know what rainbow poop has to do with the fear of God or the cycle of abuse or generational mental illness. But every single little thing could be a reflection.

...So, what does that have to do with the Siren, really?

It's that the Siren is the only example in this game as a whole of a feminine character other than Isaac's mother. There's a lot of other characters that use feminine pronouns (barely notable), titles (Carrion Queen is not notable either), but none of them go to the extent The Siren does, which fits her title as well, her name, the purpose of a siren as one would say in mythos, and also how the religious theming in this game twists it a little to be almost succubus-esque.

Somehow, this has to mean something. And clearly as you read this, you've signed up for my awful personal interpretation of what the Siren being in Isaac's little mind palace of death and doom could represent; consider where you fight her. Consider the alt path that leads up to the Mother fight as a whole, even if it's not mandatory to go to the Mausoleum to do just that, but it's necessary to do so. The Mausoleum is largely occult-themed, the type of aesthetics that you would see Christians fearmonger about during the Satanic Panic of the 80s; people in robes chanting in tongues and summoning Satan. The second floor of Mausoleum is always a more powerful version of the ever-iconic fight with Mom. And for the Alt Path, the final stab to kill her. In the original Antibirth mod, this was taken very literally, the path was Isaac just flat out killing her. This isn't canon anymore of course, and IMO the new Mother ending is a lot more heartwrenching and fitting, but it's kind of notable here. Everything on this third alt path is meant to be soaking in sin and letting it control you.

Not to mention the Gimmick of The Siren. The familiars that look to be babies, or as young if not younger than Isaac, are typically referred to in a friend-esque manner. They're Friends, friends that get turned against you, which also has a slight parallel with the increased isolation and deprivation Isaac's Mother put him through in the real world as a result of the increasing amounts of religious illness.

These all pile up to my own sentence and ultimate idea of what the Siren represents, which is Isaac's thoughts on if he had a different mother, perhaps a kinder one, but different, not her, and how that in itself, wanting to separate from the blood of family, would be considered one of the highest sins.

So yeah, that's a whole lot of scheming over a boss fight that usually lasts less than two minutes. But hopefully you understand my point here with how she just makes sense to be there.

BONUS ROUND from my friend Val who also helped proofread this part, thank you so much btw;

"Sirens in greek mythos actively weaponize not physical strength or form (initially) but make you mistrust your senses and perception of what is reality. They would promise endless joy and passion and all of these things to you and your friends; in a way, The Siren NOT working on Isaac is a pseudo-reflection on how he has been burned by all of these promises that his friends (familiars) fall for and in a way he's got the 'beeswax' (as in the odyssey) in his ears that allows for him to not fall for her tricks."