Morfin was born to Marvolo Gaunt and lived with his father and sister Merope. They lived in squalor and Morfin was raised with Pure-Blood supremacist values. He was a parselmouth and enjoyed talking to and killing snakes.
Morfin was born out of many generations of incest in order to preserve the Pure-Blood line and was mentally deranged as a result. He was violent and had a history of attacking muggles. He also enjoyed tormenting his sister Merope.
Bob Ogden's Memory
Harry and Dumbledore explore Bob Ogden's memory, in which he is sent to charge Morfin with attacking the muggle Tom Riddle Sr. Morfin warns Odgen to leave but Ogden did not understand as Morfin was speaking Parseltongue, so Morfin hexed him.
Marvolo intervenes and Odgen explains Morfin's charges. Morfin gleefully tells them that he had seen his sister watching Tom Riddle and so attacked him and caused him to erupt in highly painful hives. When Marvolo hears this he attacks Merope and Odgen tried to stop him. Marvolo and Morfin both attack Odgen, causing him to flee to the Ministry of Magic for reinforcements. Morfin and Marvolo try to fight the Ministry wizards and are arrested. By the time Morfin gets out of prison, his father had died and his sister had run away with Tom Riddle by enchanting him with a love potion.
Tom Riddle Jr. (Voldemort) tracks down the Gaunts in order to learn about his family. Morfin sees him and thinks he is Tom Riddle Sr. and attacks him, but Voldemort stops him by speaking in Parseltongue. Voldemort asks about the family and Morfin explains that Merope ran away with Tom Riddle Sr. but he left her. Voldemort then stuns Morfin and modifies his memory. Voldemort kills Tom Riddle Sr. and his parents using Morfin's wand, steals Morfin's ring (which is the Resurrection Stone), and makes Morfin think that he committed the murders.
Morfin is later arrested for the three murders and happily confesses. Years later, Dumbledore tracks him down and uses Legillimency to extract the real memory. However, Morfin dies in Azkaban before Dumble can secure his release.
Even though Morfin dies before the series starts, he plays a major role. Morfin, by abusing his sister, led to her using a love potion on Riddle Sr. and choosing to die when he left her. This plays a major role in Voldemort becoming who he was. Morfin told Voldemort about his family history, which contributes to Voldemort's hatred of Muggles and the murder of his father. Voldemort is then later able to use his father's bones to resurrect himself. Morfin provided Voldemort with the Peverell Ring which Voldemort used to make a Horcrux. The ring was also the Resurrection Stone, so Dumbledore tried to use it to see his family, which led to his death. Before Dumbledore died, he sent the ring to Harry, who used it to talk to his family and get the encouragement he needed to face Voldemort.
- In some ways, Morfin Gaunt can be easily considered to be the Greater-Scope Villain of the Harry Potter series: if he hadn't attacked Tom Riddle Sr., then Bob Ogden would have never been send to the Gaunt residence and Merope would have remained living with Morfin and Marvolo, precluding her from seducing Riddle and conceiving Lord Voldemort with him.
- However, Morfin wasn't the sole responsible for Voldemort's existence: had Bob Ogden decided not to return with reinforcements, then Merope would have possibly never left her family nor had the opportunity to seduce Tom Riddle Sr. Likewise, according to J.K. Rowling, if Merope hadn't lose her will to live and raised Voldemort on her own, then Tom Riddle Jr. would have turned out to be a better person.
- The flashback scene featuring Morfin and his family was cut from the film adaptation of Harry Potter and the Half-Blood Prince because of time and pacing concerns. However, it was originally present in an early draft of the film's screenplay according to director David Yates. It's unknown if there were any considered actors to play Morfin by that point.