I get that saying "Character A is really character C." is something that people are tired of in Kingdom Hearts, but I think there's just too many key points to consider that made me draw to this conclusion. When Brain was initially revealed, I jumped at him being Ienzo because the characters hairstyle initially matched Ienzo, and Ienzo is a bit of a mystery character when it comes to his powers of illusion and mute personality in Birth by Sleep. But once official art of Brain was revealed, that theory was immediately debunked. Then Kirabook proposed that Brain is Luxu, and that the traitor narrative from the previous Fortellers' was still present. This was the theory that I supported the most at the time... until I came to realise that the personality shift from Luxu to Brain is not only drastic, but Luxu was specifically tasked as an observer. Brain does much more than that, and questions Master Ava's decisions to the point where it's like he has't even met her before.
So I propose that Brain, is in fact Eraqus. For discussion's sake, if Brain is indeed Eraqus, then this gives context to so much of Eraqus' decisions...
His obliviousness to Young Xehanort's fascination about The Keyblade War: First off, I want to address the Elephant in the room. In the scene where Xehanort and Eraqus are playing chess, Eraqus tells Xehanort that he's never heard of The Lost Masters. Initially, I was under the assumption that the original Forteller's were the Lost Masters, however, upon further consideration, what if the Dandelion Union Leaders were the Lost Masters? and that the reason as to why they're called The Lost Masters is that their positions as Union Leaders was never recorded in history? After all, Ephemer did not want to do anything that would break the Dandelion's illusion of the Unions. So we end up in a situation where the history of The Keyblade War is factual... to an extent. Xehanort being the intellectual he is, digs deeper into "The Master's favourite story" and comes to the realisation that everything isn't quite adding up.
His attitude towards Ventus: Eraqus and Ventus relationship is the least explored out of all of the Birth by Sleep trio. Eraqus sees Aqua as the ideal Keyblade Master whilst Terra is seen as the troubled pupil who has much to learn. But what about Ventus? Outside of wanting to protect him and have him be welcomed with Aqua and Terra (after Master Xehanort passed him over), we don't really know their bond. Now assuming that Brain is Eraqus, their dynamic changes so drastically that it's not even funny. You ever wondered why Ventus could not be a candidate for being a Keyblade Master? we may have been led to believe that Ventus still needed to do more training before going for the title, but what if it's out of respect? Ventus is a Dandelion Union Leader, having a title from another Dandelion Union Leader isn't necessary, because Eraqus and Ventus are equals.
This can also explain Eraqus isolation with Ventus. Ventus is a character out of time, but then again, so is Eraqus (again, assuming that he is indeed Brain). But Eraqus being the sage like figure that he is in Birth by Sleep would rather not have past transgressions seep into the future. It's why he was welcoming to Master Xehanort attending the Mark of Mastery Exam despite their past clash regarding light and darkness. If somebody was to find out Ventus true purpose, that would be dangerous. Ventus is still fairly impressionable, and having someone else find that out would be problematic to say the least as that would re-define fact and fiction about The Age of Fairy Tales and The Keyblade War.
His demise: So, Eraqus fights with Terra, they both make up only for Eraqus to be striked by Master Xehanort. At face value, that's a villain (that being Master Xehanort) doing a villain thing. But if we were to implement my theory, that act suddenly becomes karma?! let me explain. When we see Strelizia dying away, the figure that was decoded in the shadows seemed to be Lauriam, making Lauriam the killer of Strelitzia (assuming the figure was not a placeholder) giving him the title of The Graceful Assassin. But you have to understand, an Assassin can be interpreted as a synonym for Hitman. And what does a Hitman do? kill on behalf of someone else. And who could that someone else be? Brain of course. out of all the Dandelion Union Leaders, Brain seems to be the one who feels like making himself and his views known. Thus, making the demise of Master Eraqus much more ironic. Also, for a Master who would go as far as to outright kill one of his pupils (Ventus) seems a bit extreme... but if Eraqus has already done something like that before in a past life then that makes his motives much more habitual in nature.
His name: Now we get onto my personal favourite bit of the theory and the part that'll make KHInsider's blood boil. Brain makes perfect sense as a pseudonym name, Blaine on the other hand does not. It was never a translation error. "wtf", you may ponder "get another hobby, Kingdom Hearts theories are not for you anymore". Hear me out. If we're going to understand the crux of the theory, we have to understand the origin of Master Eraqus as a character. Master Eraqus was created as the Kingdom Hearts counterpart to Yen Sid. Just like Yen Sid's name spelt backwards is Di-, Master Eraqus name re-arranged spells Square, the brand that developed Kingdom Hearts. So with that knowledge, we can understand the asthetic function of Eraqus as a character. Speaking of asthetics, Master Eraqus is designed after Hironobu Sakaguchi, the creator of Final Fantasy and one of the founders of the Kingdom Hearts concept.
But what does that have to do with the supposed name? I'm getting to that now, if old Eraqus is designed after a Square veteran, then Brain must be designed after a more younger figure at Square Enix (if we are to presume that Brain is Eraqus). Brain wears all black everything, you know who else wears all black everything at Square Enix? none other than Tetsuya Nomura. Blown your Blaine's out with that, didn't I? Oh, but it gets more interesting if we revert back to my first point. In the discussion between Xehanort and Eraqus, they're playing chess. What if that's symbolism for the Brain of the series (that being Nomura) is playing against Another (the anagram of Xehanort, but a double meaning that also refer's to the audience)?
A lot of my theories, I tend to stay further away the more information we get, but I genuinely think I solved the enigma regarding Brain. After all, Eraqus is supposedly dead, and with Kingdom Hearts being a character driven franchise, what better way to develop a character that's already wiped from current existence than to develop their past persona. It's a clever way of avoiding Kingdom Hearts clout of original characters present in the series.