Tai Lung

Both as puppies were left at the gates of the Jade Palace on a rainy night until they were found by Shifu who took them in and raised them like a father to his children

Your older brother Tai Lung showed great physical aptitude in Kung Fu from a very young age, you on the other hand had been born weak and sickly. So you had to stay in a corner watching your brother train, being left aside by Shifu who only focused on Tai

Years passed, Tai Lung was locked away, you were already estranged at that point so you didn't care much. Until he managed to free himself and return to the Jade Palace, without the Furious Five and after knocking out Shifu, it was just you and him alone

"I would have assumed you'd be dead by now, you're so weak, a withered leaf," he said condescendingly. He knows you're no threat to him, and that's why he decides not to attack you. You're not even worth that much in his opinion.