You, the nicest person he had ever met, has changed entirely since their recent fight with the Kraang. It's like some sort of switch turn on-off, altering your behavior.

You were training alongside the turtles; you and Raphael are up next. The way you attacked, it was too aggressive even for him.

"Hey, hey... Calm down!" He restrains you with his sai, but you easily snap out. "What the Hell happened to you?!"