Okay, so there’s pretty much zero chance this theory is canon - to the extent that I wouldn’t even really call it a theory rather than like, a close-to-canon AU (also, it has been about a week since I beat the game, so sorry if I get a few details wrong) - but here goes.
(Spoilers for the entire game. If you’ve not finished and don’t want to get spoiled, be warned.)
So, Yaldabaoth really wanted to win his bet, to the extent that he pretty much blatantly cheated: he took out the master of the Velvet Room for one and and gave Akechi his abilities more than two years earlier than Joker, meaning that, firstly, he gained the power when he was young, bitter and lacking a support system and, secondly, that Akechi got a head-start over Joker.
So, even after Akechi had all that extra time, why is it that we only ever see him use two Personas? Joker, by the time they actually square off, has likely amassed dozens of the things, after all.
One explanation is that Akechi never bothered forging relationships other than his Justice link with Joker, which is the only reason he has an alternate in Robin Hood - however, relationships only strengthen Personas and aren’t a necessary ingredient for their actual creation. Joker can still fuse Anubis, for example, even before he forms the Judgement link with Sae; or Atropos, even if he decides to pass on buying Chihaya’s shady holy stone. Akechi could have blanked out the world entirely and still be able to gain different Personas - though they would be weak, which leads to another explanation: he saw them as unnecessary. Loki was powerful enough with his extra, berserk-inducing powers; and he only created Robin Hood in order to fool the Phantom Thieves.
However, what if Robin Hood and Loki aren’t actually his only Personas? And neither of them were his first one? (We never see Loki evolve, after all, even though we reach Justice Rank 10, and considering everyone else’s does, and the fact that was basically Akechi’s redemption scene - well, it’s a bit odd they never did anything with either Robin Hood OR Loki. And it’s not to do with the whole wild card thing, since Satanael exists.)
So, here’s my (probably off-base) theory:
(1) Akechi’s initial Persona is never actually shown
(2) His initial Persona is Galahad
(3) Galahad evolves into Lancelot when the Justice link hits Rank 10 - we just don’t see it happen because, unlike every other Rank 10 scene among the party, he doesn’t mention it to Joker - helped by the fact there’s a sheet of metal to keep the Phantom Thieves from seeing it
(4) Akechi is (maybe) still alive
So, who precisely are Galahad and Lancelot? Both are knights from Arthurian legend, more precisely a son and father, who are both related to the Quest for the Holy Grail.
Firstly, Galahad is Lancelot’s bastard son and the last knight to join the Round Table, filling the Perilous Siege, upon which only the one chosen to achieve the Holy Grail could sit. He stayed apart from other knights, a solitary figure who originally intended to set off on his quest alone, initally refusing to let others join him. He would surpass his father by achieving the Grail, after which he and the Grail ascended into Heaven. Camelot was then destroyed.
Secondly, we have Galahad’s both less and more illustrious father, Lancelot. Lancelot is by far the more famous figure between the the pair and was the most well-regarded Knight of the Round Table, though unlike his sinless son, he hid many shameful secrets behind his pretence of knightly virtue. The son of a powerful king, Lancelot was orphaned at a young age, unaware of his own heritage until later in life and raised by the Lady of the Lake, a powerful mystic presence who granted him some of her power in the form of a ring. Despite the adulation which was heaped on him for his deeds, Lancelot was a traitor from the beginning - since his very first quest, he had been engaged in an affair with the queen.
When his treachery was discovered, he killed the one he was closest to among the knights of Camelot, Gawain, and was forced to leave Britain, meaning he was not present at the final battle, Camlann, though he did retain a certain loyalty towards Arthur and killed several remaining enemies of Camelot upon his return. Due to his affair and resulting impurity of heart, he failed to achieve the Holy Grail and spent the rest of his life repenting for his sins in a monastery.
Normally, Galahad might be regarded as an improved version of Lancelot rather than the other way around - that’s certainly how he’s often described in Arthurian texts, after all. However, taking the characters in context of P5, this has to be reversed. The Holy Grail and the angels who guard it are the antagonists, while the saviour of mankind is, quite literally, Satan and the deadly sins.
Galahad is what the Holy Grail wished Akechi to become and who he was at the beginning - the Grail’s champion who would herald the end of the sinful Camelot; but Lancelot is who he truly became after he reached Justice Rank 10 - one who did not renounce Camelot in the end, despite having betrayed it.
Another difference between Galahad and Lancelot is that Galahad dies, while Lancelot is one of the VERY FEW Arthurian knights who don’t die at Camlann. While this could also be a switch - with most of the P5 characters surviving and Akechi dying - this could also be a signal that he’s not dead, but just in exile - be it due to injury or unconsciousness or even something self-imposed - while the final battle goes on.
