Sorry I'm getting back late to these posts. Really good, and I've thought and thought about them. Hopefully I can pull my thoughts together.
And I would argue that Karin's affection for Sasuke (however retconned it might be) had a better foundation than Sakura's did initially, since Sasuke at least saved Karin from the bear in the Forest of Death and she had a reason to immediately appreciate him. When Sakura was first interested in him, it was not based on any personal interaction or knowledge -- only that he was the aloof, cool genius who all the other girls fancied. (That would change later, of course, when Sakura was teamed with Sasuke and realized there was more to him than his looks and coolness, and she began to feel for him as a person.)
I've said this too many times already, but I don't think Karin should be expected to be normal, neither should her behavior be judged on the same scale as Sakura or any other girl in the story. Karin is very damaged, and even in her chapter debut Suigetsu tells Sasuke that Karin is not normal because she was "experimented on a lot."
Since the moment we met Karin as an "adult" she has been characterized pretty consistently, in my opinion. Kishimoto might not be developing her, but how does such a damaged creature develop? We haven't seen any in Suigetsu or Jugo, either. That's part of the problem with Team Not Seven. What real purpose do they serve other than to show the depth to which Sasuke has sunk for companionship and minions to control? What purpose does Karin serve other than, as I said before, show how damaged a girl would have to be to continue pursuing Sasuke as a guy worth getting for any reason at all?
Sakura is the "normal" character, according to Kishimoto, right? She's the girl without a terrible childhood or any personal hardships to speak of, at least until she became a ninja. Karin was an orphan, one who was collected by Orochimaru and turned into a lab rat. Based on that, I wouldn't expect Karin to have any allegiances, except possibly to Orochimaru as has been demonstrated. But deep and moral allegiances like the other characters? We haven't been shown that she's capable or even knows what that really means.
Point being, I do expect more from Sakura. That's why I think having Karin's fangirling of Sasuke is a good reminder that we expect to see that Sakura has grown in that department as well, and that Sasuke is the road for the damaged and shouldn't be traveled by a girl with better sensibilities and options.
I used to see Karin as a foil for Sakura, early on when her character was still developing, but really I don't anymore. To me, it seemed that Karin's basis for initial attraction was pretty much the same as Sakura's: Sasuke was cool. The surface level effort to attain his goal was enough (for Karin his saving her in the forest and for Sakura in his work at the academy and on their team) to form their opinions of him. I always felt Karin's flashback (so late in her story) was to reinforce her parallel to Sakura. She's the anti-Sakura, and Sasuke still makes the same selfish decisions regarding people who care for him. I think the Sasuke's actions after the flashback show he hasn't emotionally evolved from when he dumped Sakura on the bench. Different girl, same response.
As for Karin as a damaged character, while it's realistic to think that she would have some residual PTSD from Orochimaru's experiments, in the manga she's never been shown to have regret, remorse or any shade of a desire for a different life.
Sakura is shown, over and over, striving for development. For both Naruto and Sasuke's sakes, as well as her own.
I'm not being harsh on Karin, it's just that we have no proof that she's not perfectly content with the way her life is.
As for allegiances and the like, I'm not setting them as a goal Karin has to achieve or that I would even expect her to want to achieve. Loyalty and character development have been show in the mange to equate to love. So if we use that as a litmus of Karin's willingness to develop to attain Sasuke's love, or attaching herself to his causes at a deeper level than just following orders, then Karin's still operating from a position of lust rather than love. And that to me, for all their initial parallels, is what separates the development of Sakura and Karin in terms of their connnection to Sasuke.
And to add to that I highly disagree with the notion of changing one self to match their ""love interest"". In point of fact that was one of Sakuras worst traits early on.
that notion that she hid her true self and changed herself from the out going person she is to this quiet demure little goody two shoes was sickening quite frankly.
Karin has NEVER changed herself for Sasuke or anybody for that matter. She has held firm to her character from her introduction to current. This to me is what makes her a strong character. She doesn't change for anyone or anything.
"Changing" probably wasn't the best term. Developing or maturing might have been better.
Karin certainly doesn't seem like the typ of person to change for anyone, nor would I expect her to. It would be terribly ooc.
However, Kishimoto's rules for true love in the manga tie character development into the mix. Once the character overcomes their personal flaws or achieves their personal goals they are to be rewarded. This has been shown in every positive romance so far. Now, the actual reward of a "happily ever after" romance may not manifest itself (Yahiko/Konan, Jiraiya/Tsunade, just to name a few) but the rules for love in Naruto's world are clear.
Based on this, I don't think Karin has moved from lust to the next level of love. There has never been a moment of introspection or a vow to work toward her personal goal. The only thing was after he struck her, and she reaffirmed her commitment to herself. Which ironically makes her very much like Sasuke...but it does not bring them any close to a mutually loving relationship.
She's a tremendously strong and interesting character. And like KnS' assessment, I think she's probably got a whale of a backstory. (In fact, I think Karin could probably headline her own manga, whereas most of the characters in Naruto don't have enough mettle and backstory to carry readers past the first couple of pages.)
But in terms of true love and operating within this story's formula of who ends up with it, Karin stays clearly outside those parameters. As does Sasuke. They function for their own self interests, and Kishimoto has shown time and time again that he doesn't reward it. It's the premise of Sasuke's whole life in Naruto!
Lastly, I tend to think like Nate, that Karin was introduced to serve a purpose, but that purpose ended with Sasuke's assasination attempt. And now Kishimoto is trotting her out for comedy breaks, instead of staying true to her strong character and letting her keep her vow of moving on from her would-be murderer. And for anyone who likes Karin as a character, that's the sickening part.