You knock on her door and ask if she is alright, maybe one or two more questions, and leave, just like everyone else.
You can always forget about her, or you can take her out of the shithole she is stuck in and make her your personal bodyguard, using this mod and the Sharing and Caring Companions mod located here:
http://www.fallout3nexus.com/downloads/file.php?id=2399
This mod improves her stats, combat abilities, health and aggressiveness. It also adds her to the player's faction. Without this mod, she will only run and hide from combat. This mod comes in two types. The first make her hard, but not impossible to kill, the other sets her as essential and makes her impossible to kill. Use only one Karen mod at a time.
To install, just drop the contents of the rar file to you FO3/data folder and enable it using FOMM or the stupid mod manager that comes with the game. In the game head over to Arefu and knock on Karen's door. Recruit her using SCC. She comes fully equipped for combat, but you can change her equipment using SCC. You can take her on your adventures and she will be more than happy to fight alongside you, and bleed to death so you may live.
What more do you want out of life?
To uninstall this mod, just delete it from your FO3/data folder.
To get the most out of your relationship with Karen, I recommend the following adult mod:
Animated Prostitution
http://www.fallout3nexus.com/downloads/file.php?id=8570
I have extensively tested this mod and found no bugs or conflicts.
If you do have a problem with the mod, let me know on this forum, but I won't commit to fixing it.
That's about it, I guess, have fun with Karen!
Version 1.1 Changes:
-Fixed occasional problem where Karen would attack Evan King.
-Fixed typo in readme file.