Definition: In this task, you will use your knowledge about language (and common sense) to determine what element the marked number refers to. The numbers are marked with two underlines around them, like: _ number _. Your answer should be chosen from the given text, and should not contain other words.
Input: Kate Beckett:  The moment that I met you , my life became extraordinary . You taught me to be my best self , to look forward to tomorrow 's adventures . And when I was vulnerable , you were strong . I love you , Richard Castle . And I want to live my life in the warmth of your smile and the strength of your embrace . I promise you I will love you . I will be your friend and your partner in crime and in life , always .
Richard Castle: The moment we met , my life became extraordinary . You taught me more about myself than I knew there was to learn . You are the joy in my heart . You 're the last person I want to see every night when I close my eyes . I love you , Katherine Beckett . And the mystery of you is the _ one _ I want to spend the rest of my life exploring . I promise to love you , to be your friend , and your partner in crime and life , ' till death do us part , and for the time of our lives .

mystery