Andre Barrett was the boyfriend of a clinical trial patient named Greta. Barely anybody ever saw them together and many assumed he did not exist.
When Greta called and told him she was having an operation, he asked her to wait three hours until he could come down and talk to Dr. Shepherd and ask him some questions.
However, he arrived too late and Greta was in a coma, from which she never awoke, after the surgery.
Andre met his girlfriend, Greta, on a cruise, they didn't take any pictures and nobody who knew Greta had seen them together so people assumed he wasn't real, that he was merely a creation of her tumor.
Andre was on a cruise around the Greek islands, they made a stop somewhere and when the ship was about to leave again, Greta ran, but left her shoe behind. Andre found the shoe and returned it to her at dinner. The story is quite similar to the fairytale Cinderella.