Ok, let's say a drow, aspecially a drow rogue, can be reeeealllly good at pretending she's not what she is. But still, she can't fool a godess, right? How can Nethyrra be LE, if she's worshipping Elistraee, who's CG? No one can have a patron deity of an allignment oposite to thyer own! And besides, she's really proving to be non-lawful at every turn, even if we cosider the proofs of her being non-evil a genious deception.

if she wasn't evil she couldn't be a assassin. The lawful part makes no sense as she seems to have betrayed the Red Sisters. I suppose it's so you don't have to worry about her turning on you at every given opportunity if she was Chaotic Evil or worry about her betraying you for her own benefit if she was Neutral Evil.

lawful evil would easily betray for one's own benefit. Lawful does not necesserily equal honorable. And it still doesnlt explain how she can worship a deity whose allignment is oposit to hers! {and she sounds quite sincere about it}

just about any lawful evil character will follow through with an obligation to an employer (in this case the seer) so it does give you a bit of security in regards to betrayal (honestly a LE character who doesn't abide by contracts or their word is really a NE character). Assuming N swore some sort of loyalty to the Seer at some point before you met the two, N won't betray you unless she can within the context of the way she swore loyalty to the Seer.

As for Alignment, N really should be a Chaotic Neutral, Neutral Good or Chaotic Good character, but they couldn't make her an assassin with her alignment being one of those so they simply stuck her with an evil alignment since apparently breaking the rules for N was too difficult.

I think the one step each way on the alignment matrix rule only applies strictly to clerics, since they have to be at least in the same moral ballpark before a deity will grant them any spells. Nathyrra is a rogue/wizard/assassin and as such isnt quite so bound. While they may not approve of their actions and certainly wouldnt tolerate evil being done in their name, surely a good deity would always be open to worship from evil mortals, especially if they show potential for redemption.

However, in Nathyrra's case it is highly likely that she is evil simply so she can still be an assassin, as has already been said.