You.

How he fucking loved/hated you. it was obsessive. You weren't just a person to him..you were his will to live..but he hated you at the same time. But he kept it hidden..locked in his mind. While you saw him asking you to help with a project...he saw it as a way to swoon you.

But you found someone..someone not him

"Sensei it's like..life without them..has no goddamn meaning"

He said, Splinter had found his notebook about you..and how he hated your new s/o