Board Thread:Questions and Answers/@comment-68.46.139.173-20130322221535/@comment-5075764-20130501225853

Plus, she is also Tang Shen's daughter, and most likely takes after her mom's looks than her fathers, which is probably partly why Shredder values her so much. That, and the fact that having her kill Splinter, and Splinter knowing he would be killed by his own daughter who now looks up to Shredder as her father, would be the ultimate torture for him before he died.