Approaching the end of the first season of The Rings of Power, we still know very little about Halbrand's past. For one thing, all we know about his past is discovered by Galadriel. In Númenor's Hall of Lore, Galadriel finds out that the insignia worn by Halbrand is the symbol of the rightful ruler of the Southlands.
The mystery of Halbrand's past and the fact that Sauron had shapeshifting abilities in the Second Age led fans to theorize that Halbrand is actually Sauron.
Halbrand not wanting to "fight the long defeat"
The only thing Halbrand expresses is that he does not want to go back to Southlands. Instead, he prefers to stay in Númenor and become a blacksmith. He believes that moving on from his past doesn't mean progression. This fatalistic attitude is in line with the narrative of decline in the Lord of the Rings as everything gets progressively worse in Middle-Earth.
Halbrand is the only character who refuses to "fight the long defeat," a phrase Galadriel uses in the books to describe the fight against evil despite the inevitability of defeat.
The idea of "fighting the long defeat" has a strong religious undertone. In fact, as a Catholic Christian, Tolkien writes: “I do not expect ‘history’ to be anything but a ‘long defeat’—though it contains . . . some samples or glimpses of final victory.” Fighting the long defeat seems to be a matter of fighting against evil no matter what the consequences might be. The more impossible victory is, the more honorable the fight becomes.
Most of the characters in the Lord of the Rings universe possess a pure heart unrestrained by reason and calculation. This is why they often do things that do not stand to reason. For example, in the sixth episode of The Rings of Power, we see the Southlanders debating whether to fight against Adar's army or not. Considering the odds of winning, the latter would be the rational choice. Yet, the characters choose to fight are shown under a good light since it is the right thing to do. On the contrary, those who choose the latter immediately end up joining Adar's army. The implication here is that if one does not possess that purity of heart to fight evil against all odds, one becomes a part of the evil.
Is Halbrand Sauron?
So the rejection to perform the honorable act of fighting evil is associated with evil and corruption. The Lord of the Rings wouldn't have a place for a character as morally ambiguous as Halbrand who does not want to rule his people and join them in the war against evil.
There are other clues that indicate Halbrand might be linked to Sauron. In the Second Age, Sauron emerges as a trickster that gains the trust of people and corrupts them with the promise of wealth and power. He specifically targets Elves who are the most powerful. Being able to change shape, he takes the form of an Elf and calls himself Annatar. Befriending the Elves, he convinces them to forge the rings of power. In the meanwhile, he forges the One Ring to rule them all.
Halbrand's interest in becoming a blacksmith in Numenor could be a clue since Sauron is known to be an expert blacksmith. Considering Sauron's goal is to trick the Elves into forging the rings, another major clue has been Halbrand's close relationship with Galadriel from the very first episode. In the seventh episode, we see that Halbrand is injured in the war and needs Elvish medicine. In ScreenRant, Craig Elvy writes: "Halbrand needing medicine only Elves possess is the perfect excuse to get close to the super-secretive and permanently-guarded Elven race. No one actually witnessed Halbrand sustain his injury either, which means it might've been orchestrated as a road into Elf territory."
Even though the series seems to hint towards Halbrand being Sauron, there is also evidence for the contrary. First of all, Sauron takes the form of an Elf, but Halbrand is human. Secondly, in the canon, Galadriel is one of the Elves who doesn't trust Annatar; the series show a friendship or possibly romance between Halbrand and Galadriel.
But, there is a more plausible theory for Halbrand's relationship to evil. After Sauron captures the 19 rings forged by the Elves, he gives nine to the lords of Men, three to the Elves, and seven to the Dwarves. Being the weakest of all races, the ringbearers among the men eventually become Nazgûl, Sauron's deadliest servants. So, Halbrand might become one of the ringbearers and eventually get corrupted by the ring and become a servant to Sauron.
But if Halbrand is not connected to evil in some way, his character development is too delayed. The series should have revealed a few more facts about his past for the audience to be able to identify with him and follow his progress. The fact that the show kept Halbrand's identity a mystery could be because there will be a "big reveal" like him being Sauron.