Aizen x Hinamori (or Hitsugaya x Hinamori, I suppose): "Girlfriend in a Coma" by...uh...not sure who originally. I have a cover by a band named Bleach. (Ha!)
Girlfriend in a coma, I know, I know - it's really serious

Kaien x Rukia: "For Reasons Unexplained" by Casey Stratton
I come to you
But I am not the same
You sense my fear
Tell me it's ok
A cross to bear
I pull too far away

Ichigo x Rukia: "Hollow" by Casey Stratton (hur hur hur)
It's just as well
You could always tell when I was hollow

Aizen x Hinamori: "The One That Got Away" by Natasha Bedingfield
Got so many questions feelings I can't explain
We're worlds apart don't even know your name
I'm longing to give you my heart

Ichigo x Orihime: "I So Wanted" by Rie Fu
I so wanted to be in your heart
But you wanted her and me on my own
You don’t know that it left another hole

Renji x Rukia: "The Investigation" by Sugarcult
This could be perfect
It could be worth it
This could be perfect
If I wasn't worthless

Wow, I had more than I thought.