She said that she would like to have the person of her heart and we will never be apart until we grow old together.
He said that he is willing to win the heart of one person and stay together forever.
She said, you remembered wrongly, a Yixinren is a person who stays with you wholeheartedly for a lifetime.
He didn't say anything, just pursed his lips and smiled. You said "one heart" because you don't know who can be with you for life; I said "one heart" because I know very well that what I did was just to win your heart.