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Lonely Karliah

Karliah is marriable and can be a follower. Compatible with Hearthfire but doesn't require it.
Nightingale Hall is too cold and dull for such a beauty, she feels so sad and lonely. Who will make her happy? Dead Gallus? Doubt it.

Finish the Skeleton Key storyline. Note, it ends when you return to Brynjolf from the Twilight Sepulcher and tell him about the Key. Then go to Nightingale Hall and speak with Karliah. She should have follower lines right away, for marriage lines you must be wearing an Amulet of Mara, like with any other regular candidate for marriage.
Give her flowers and she will take off her hood. Can be done via follower's "trade things" dialogue or reverse pickpocketing.

Features.
. Karliah has follower's and marriage dialogues.
. You can divorce her and remarry.
How to divorce: throw The Bond of Matrimony out of your inventory (all of them), dismiss Karliah if she is your current follower, talk to her and say that you want a divorce. WARNING. This option works for vanilla marriage (resets marriage quests). If you are using other mods altering marriage system it might break them or your other marriages. So if you want to divorce - use the divorce option of the mod you got married with.
. Compatible with Hearthfire. It mostly concerns the divorce option.
. Karliah sees when her Dragonborn spouse takes off the clothes, comes near and can be undressed too. If there happens a bed (or anything suitable) nearby she will lay in it. (No new textures or animations added.) It's disabled by default. To enable it unequip your clothes, talk to her and ask if she notices anything. For her not to notice you naked ask her not to stalk you.

Unobvious features.
. After the wedding she begins to buy stolen items.
. Whenever Lover's Comfort is being added to you, it is being added to her as well.
. Her morality is set to Violence Against Enemies, so she will steal things when you ask her. But she cannot steal and sneak at the same time, be careful.
. Karliah and guests don't leave the wedding ceremony until it's finished. Thieves Guild members and Maven can come to the wedding as her guests. For it to work, please, walk through the front door of the Temple of Mara (in or out) after have arranged your wedding with Maramal but before you enter the Temple of Mara to attend your wedding.

Uninstallation.
Divorce Karliah if she is your wife. Dismiss her if she is your current follower. Leave the location where she is. Save the game and disable this mod in Skyrim's launcher.

Details and cheating.
The main quest of this mod starts working when you enter a location with Karliah. So if nothing works try to reenter that location (pay attention that Nightingale Hall location doesn't end upon exiting the cave, it's safer to go inside Riften and return back to be sure that you actually changed location). You can force this quest to run (even before the completion of the Skeleton Key quests, but be warned: in this case she might have follower/marriage dialogues and not follow/come to wedding) - try this when you are in the same location as Karliah:
SetStage LKKarliahNeedsLove 0
or if the previous line doesn't work:
SetStage LKKarliahNeedsLove 10
For manual divorce:
SetStage LKKarliahNeedsLove 85
For manual uninstallation (if she is your wife divorce her first, then type this line):
SetStage LKKarliahNeedsLove 100

Descriptions of changes for all versions are available in the Chage Notes tab.

On Nexus http://skyrim.nexusmods.com/mods/16002

Version for Skyrim Special Edition:
on Bethesda.net https://mods.bethesda.net/#en/workshop/skyrim/mod-detail/3328699
on Nexus http://www.nexusmods.com/skyrimspecialedition/mods/6354