In 'Flowers for Algernon' the love between Charlie and Alice Kinnian is what?

Sad and sweet. He had fallen in love with his teacher, but for reasons of his stunted emotional growth could not properly pursue the relationship, or when having it, consumate it.

Near the end, as his IQ was dropping, he was able to consumate it, though. It was rather bittersweet, as both of them knew that he would soon be retarded again.