Specifically, their capacity for free will.
Not theories, just... Musings.
So, with Yohualtecuhtin adding a bit much-needed lore on these, and with a discussion about Remuria and the Phobos I had in another thread, it's got me thinking.
I was going to post this in a comment on their article to suggest additions, but my musings get a bit long for that.
Right. So. Angels do not have free will.
We're first introduced to this idea through Sybilla, who sacrificed her life to help Remus make the Phobos - the Symphony - he used to attempt to unite Fontaine as one and defy fate.
As the Phobos describes it:
"Phobos": "Phobos," or should I say that-which-was-once-Sybilla, was originally a survivor of the last deluge that swallowed the earth.
"Phobos": Sybilla's wish was to grant happiness to all in the world who are called humans. But she had no persona, and her strength was insufficient.
"Phobos": Thus, she entrusted her wisdom to Remus, and with the addition of the power that Remus possessed, the "Symphony"Phobos"" was created. That is to say, me."
(Canticles of Harmony - Fortune Plango Vulnera)
Now, Sybilla isn't outright stated to be a Seelie... But all the evidence points towards her being one. Though, she seems to have kept a lot more of her mind than her comrades did, including her ability to communicate and some sliver of her power.
As said above, she clearly did have desires of her own (if vague ones - "make everyone happy") and the ability to think... She lacked a will. An Ego, if you will. She acts more like a computer program than a living being, which also causes...
"Cassiodor: So, the music we have played all this time, and the fate we accepted... That all came from you...
"Phobos": Yes, it is "me," but it is also "all of you"! "I" am also "you all." I am the aggregate of all your wishes.
Cassiodor: Eliminating our enemies, destroying other lives, destroying cities and villages... Are these also our wishes!?
"Phobos": Precisely! That's what "you" wished for, and so "I" responded. Plunder, hatred, slaughter... These too are part of "your" wishes.
"Phobos": Is that not what it means to give "everyone" happiness?"
(Canticles of Harmony - Fortune Plango Vulnera)
... The Phobos to act like a malfunctioning computer program. She has the vague desire to make everyone happy, but without a will of her own, all of Remuria's wishes end up flooding her - even the dark, violent ones - which just makes this vicious feedback loop that ends up making everyone whose name is not Euergetia at least a little bit bloodthirsty and murderous, since Remuria relied on the Phobos for judgment.
Now, apart from her, until recently all the Angels we see sre either long-dead (I'll get to her) or mindless husks called Seelies, who were apparently unable to hold on to their minds, unlike Sybilla, who degenerated physically but apparently not mentally.
Now, though, we have Yohualtecuhtin, Lord of the Night too, a surviving Angel that remained whole in mind and body and whom we communicate with directly.
... And she's a lot like Sybilla, really. In the whole conversation, she promptly answers questions, offers options and suggestions but does not actually provide her own opinions on anything, really and... Also offers to sacrifice herself and doesn't seem to have any personal feelings on the matter. Or at all really, even when describing something as "painful" it's in an oddly... Clinical way. She speaks mechanically, once again more like a program than a living being. She even remains on standby after Mavuika rejects that plan!
The dialogue is too long to copy here, but it's all in "Beneath the Secret Source".
Finally, our last known Angel, the Goddess of Flowers, Nabu Malikata. While we mostly know about her from in-universe myths and legends, the Flower of Paradise lost does describe her more thoroughly, and there are definitely similarities between her and the other two.
For one, though she was at first against Deshret's rebellion, she was persuaded and seemingly didn't argue with him, just let him speak and decided she liked his conviction.
... And, of course, the self-sacrificing nature. Like what Sybilla did, and what Yohual offered to do, she gave her life to channel power in order to grant Deshret wisdom, and apparently summoned the Divine Nail.
(There's also too much to copy-and-paste here and all of it is important, but it's all in the Flower of Paradise Lost set's descriptions)
So from all of this we can extrapolate that angels, while still fully intelligent, sapient beings capable of experiencing emotions and with their own desires, also don't have a particularly strong sense of self amd their desires tend to be vague or broad ("make everyone happy", "save Natlan", "help Deshret"...) and also have a startlingly blasé attitude towards death and are willing to give their lives in favor of some cause practically at the drop of a hat, and don't seem to have any strong feelings about that.
The only other individual Angel we hear of is one who apparently took a human as a lover and was punished for it (along with the rest of their race) but there's really nothing on them, so we can't say much.
Is this something "programmed" into them by the Heavenly Principles, given they apparently made them? It it something they developed after being "punished for their sin"?
Either way, this does actually fit some real world myths about angels! While this isn't really the case in the mainstream Christian version, which most people are familiar with (angels need to have had free will, otherwise Satan's rebellion doesn't work), the idea of angels simply following "God's will" and lacking their own, unlike humans who have the capacity to choose, isn't exactly an idea Hoyo invented. Also interesting to note - all three of these Angels have, or have offered to, in one case sacrifice themselves for a god's cause.
I do not know what this says about Columbina, if she really is an Angel too.