It is later revealed that Cream first learned about K's illness when she takes what she thought were K's vitamins, but in fact turned out to be pain medication for terminal cancer patients.
Earlier in the movie, Cream asked K what his wish was: for Cream to find a good and healthy man to spend her life with.
With her knowledge of his illness, Cream thus faked falling in love with Joo-hwan, in order to appease K.
This perspective of Cream's is revealed toward the end of the movie.
The night before Joo-hwan and Cream's wedding, K found the courage to tell Cream that he loves her while Cream replies "me, too".