Disclaimer: I don't own Bleach, unfortunately for moi...
Karin was good in sports, always had been. But when it came to love, which isn't an official sport, she stunk bigtime, and was almost always the dumpee rather than the dumper. It was usually that she wasn't feminine enough, or she was too loud, or just that she sucked at being a girlfriend.
But for once in her life it wasn't that she was a bad girlfriend, because she was actually able to please Toshiro, but it was that it just wouldn't work out.
He'd admitted it to her on a walk while he and Rangiku were in Karakura for a bit, and she agreed that the visits were too much trouble.
"Besides," She'd began, "I know that you and the Hinamori girl belong together. I can see it when you look at her, or when she brushes up against you." And for once it didn't really bother her.
Hitsugaya had looked at her with a questioning look on his face.
"I'll admit, I'm gonna miss you, but I'm fine with just being friends, as long as you'll give me that much." She held her hand out for him to shake, knowing that he's not really one for hugs and sappy moments, then roped him in for a hug that had no contact other than hands on eachothers' shoulders.
"Thanks. I wish you the best," Hitsugaya had said, just before his phone beeped with a warning from Rangiku to get his butt down to where the gate was so that they could get back to Soul Society.
This was just a look into the actual breakup of Karin and Hitsugaya, in my opinion. I popped into my head while listening to Night Moves, by Bob Seger, for some odd reason.