Tl;dr: Lifeline betrayed Loba.
We don’t know who wrote the letter to Revenant.
We know that it’s one of the eleven legends (excluding Revenant and Loba.)
The writer drops various ‘hints’ about who they could be—little turns-of-phrase associated with each legend. Here are the ones that I found (there might be others!):
The writer calls Loba “chica”, and calls something else "Awesome"—reminiscent of Octane.
The writer uses the (French) phrase “je ne sais quoi”—like Wattson.
The writer says they expect the “men” to “fall for Loba’s crap”—like Bangalore, who bashes men in the first season 5 loading screen.
The writer calls everyone else “all idiots”—like Crypto.
The writer “isn’t affected” by Loba’s flirting “for obvious reasons”—implying they’re obviously not interested in women, which (I think) refers to Path.
The writer uses the phrase “independent variable”—reminiscent of Caustic.
The writer says Mirage is really funny—reminiscent, of course, of Mirage.
The writer calls Revenant a “felagi fighter”—reminiscent of Bloodhound.
The writer says Revenant is their “Brudda”—reminiscent of Gibby.
Now, we can assume that the letter’s writer is pretty smart—they’re A. Smartypants, after all. And it sure wouldn’t be smart to subtly implicate themselves.
So I think we can say that the letter’s writer isn’t any of the above—(s)he isn’t Octane, Wattson, Bangalore, Crypto, Path, Caustic, Mirage, Bloodhound, or Gibby.
(Note about Bangalore: she is the most obvious suspect, yeah. But in the third loading screen in the new season, Bangalore says she’ll only fight Loba in the arena—implying she won’t fight Loba outside of it!)
So, who does that leave? It leaves Wraith and Lifeline.
I don’t think it’s Wraith. Wraith was the one who pointed out the danger of giving Revenant the power of the Halloween dimension. It would be odd for Wraith to recognize that danger, but do it anyways.
That leaves Lifeline. Which, I know, is a strange suspect. But here are some reasons she kinda makes sense. Loba incentivizes Lifeline by offering revenge on her parents; but that isn’t Lifeline’s goal in the games—her goal is to fund the Frontier Corps, to make up for her parent’s crimes. Lifeline’s arc is one of redemption, not revenge. Lifeline, then, has no incentive to help Loba.
However, it’s one thing to not be incentivized to help Loba; it’s another to actively work against her. My best theory why Lifeline would work against Loba is this: character-wise, Loba and Lifeline parallel one another—both had rich parents, who earned their wealth through crime, who they’re alienated from. But they deal with that situation in radically different ways–Lifeline earns money to help others; Loba steals money for herself. Lifeline could resent what Loba represents. (And, of course, game-play wise: Loba made Lifeline’s ult practically useless.)
Help me out tho! Did I miss any of the other character references in the story? Do you think it’s someone else? What do you all think?