I suspect she is not a agent of shar at all but of Selune.
I suspect that's true and that she does not even know it. The question is whether it's been wiped from her mind by her choice, as a means of facilitating her infiltration of the cult or whether the amnesia was inflicted upon her by the cult as a punishment.
Admittedly, I'd be more surprised if we didn't get some sort of twist when Shadowheart gets her memories back, and if it didn't involve Selune in some way given the narrative potential given her virulent hatred (though to be fair, that's probably totally normal for any Sharran) and the fact that in EA SH can get a weird magic surge when approaching a Selunite shrine, but as Eagle Pursuit says there's all sorts of ways that can go and I really hope Larian aren't going to go with anything too obvious.
My own pet theory, based on the fact that SH can also get a weird magic surge in the school in Moonhaven is that she was a child in that school abducted by Sharrans. She'll make a brief comment, I think, if you pick up the Old Schoolbook from there which mentions children and their families going missing. Moonhaven seems to have been a Selunite settlement, and SH's parents could have been attached to the nearby temple, and perhaps killed by Sharrans. I reckon she might have subsequently been brainwashed and possibly repeatedly memory wiped by Sharrans and brought up in their faith, but still be a genuine Sharran as far as it goes, though perhaps one who has questioned her faith and who might question it further if she finds out more about her past. That theory would put SH at something just over 100 years old, I think, but that doesn't seem out of the question for a half-elf and Jaheira seems to only look middle-aged despite potentially having another half century on that.
Probably all totally wrong of course, though if there's anything more definite on SH's backstory that has been uncovered in datamining then I'm going to ask folk to pop it in spoiler tags.
Not to be that guy, but more than anything I’d wonder how someone can even pretend an attempt at secrecy while openly wearing Shar’s emblem on their default outfit.
Undeniably true. Perhaps the thinking (if there is any) is that Shar's signs are also secret, but even so then it feels as though we should get a religion check to identify it, or just know it anyway if we're a religious type, and be able to challenge her on it. Even if she then lies and says something along the lines of "Oh, this old thing? I stripped it off a corpse somewhere and had no idea what that symbol meant!" and then we proceed as now until she fesses up.
