Hans Ramoray is Dr. Drake Ramoray's evil twin brother. He was only mentioned once in "The One After The Superbowl, Part 1". He was invented by Ross in hope he can help get rid of Joey's stalker, Erika Ford, who believed Dr. Drake Ramoray was an actual person. After the help of the rest of the gang playing along Erika believed it and headed to Salem to seek the real Drake. She still loved Hans.
It's unknown whether Hans is actually a character on Days of Our Lives or he was just made up by Ross to get rid of Erika, although the latter seems more likely.
Dr. Ramoray has another fictional half-brother "Ramon" in Venezuela, as explained by Joey to the friends in "The One Where Dr. Ramoray Dies".
Later in the series, a twin brother of Dr. Ramoray is introduced, Dr. Stryker Ramoray who conducts a brain transplant from Jessica Lockhart, who was murdered by her daughter, in The One With Joey's New Brain.