Cedric Diggory was an extremely handsome boy with gray eyes in Hufflepuff House. In 1993, when he was a Fifth Year, he took over as Captain and Seeker of the Hufflepuff Quidditch team. He was a very good student and a Prefect.
Cedric was selected by the Goblet of Fire to represent Hogwarts in the Triwizard Tournament (GF16), which indicates that he was also well on his way to becoming a powerful wizard. He competed honorably and did well, finally tying Harry Potter for the Triwizard Cup. Harry and Cedric decided to take hold of the Triwizard Cup together and share the win. However, he was killed by Wormtail on Lord Voldemort’s command when the Cup turned out to be a Portkey. The Portkey transported them both to the Little Hangleton graveyard, where Voldemort told Wormtail to “kill the spare”, meaning that Cedric was unnecessary to his plan to be reborn that night (GF32).
Dumbledore eulogized Cedric at the Leaving Feast with the famous “Remember Cedric Diggory” speech:
“Remember Cedric. Remember, if the time should come when you have to make a choice between what is right and what is easy, remember what happened to a boy who was good, and kind, and brave, because he strayed across the path of Lord Voldemort. Remember Cedric Diggory.” (GF37)
Cedric, along with his father, Amos, and his mother live near Ottery St. Catchpole.
Very good student, Prefect, and excellent at Quidditch.
Other canon notes and references
Trying to come to terms with the horror of Cedric's death over many weeks was the reason Harry could suddenly see the invisible Thestrals pulling the carriages from Hogsmeade to Hogwarts at the beginning of his Fifth Year (OP10, Pm). Harry actually witnessed Cedric's death, unlike the flash of green light when his parents died, or the death of Quirrell which happened after Harry passed out (JKR).
Cedric's father Amos Diggory became bitter in his old age that his beloved son was "the spare", while Harry survived and became hero of the Wizarding World (CC1.6).
Voldemort's daughter Delphi confunded Amos, tricking the elderly man into believing she was his niece and caregiver at St. Oswald's Home for Old Witches and Wizards (CS3.21). She had a Parseltongue prophecy stored inside a hissing oil lamp which said: "When spares are spared, when time is turned, when unseen children murder their fathers: then will the Dark Lord return" (CC3.19). Since Cedric was called "the spare" by her father, Delphi believed that only by going back in time to save the "spare" Cedric could she create a new world in which Voldemort had never died and they could rule the Wizarding World together. She recruited Albus Potter and his friend Scorpius Malfoy to steal an Experimental Time-Turner from the Minister for Magic's Office and use it to go back multiple times and try to change Cedric's fate (CC1.19).
In one trip back with the Time-Turner, Scorpius and Albus used an Engorgement Charm to float Cedric out of the Lake in an attempt to "humiliate Cedric to save Cedric," as Scorpius put it, but the result caused an unintended future (CC2.16, CC2.20). Cedric survived as an angry young man who later became a Death Eater, and killed Neville Longbottom at the Battle of Hogwarts. As a result of his death, Neville wasn't there to kill the Horcrux-snake Nagini, and with one Horcrux keeping Voldemort alive, Harry died instead of The Dark Lord. After that, the Wizarding World was ruled by Voldemort along with his daughter Delphi, known as "The Augurey" for the winged tattoo on her back (CC3.3, CC3.5).
After saving Albus and Scorpius and defeating Delphi, Harry took Albus to visit Cedric's grave on a "beautiful hill" (CC4.15). He told Albus that he came to visit the grave often because
"I didn’t know Cedric well enough either. He could have played Quidditch for England. Or been a brilliant Auror. He could have been anything. And Amos is right – he was stolen. So I come here. Just to say sorry. When I can." (CC4.15)
Cedric is a name invented by Sir Walter Scott for a character in his 1819 novel "Ivanhoe", based on the name Cerdic, the name of the legendary founder of the kingdom of Wessex in the 6th century. Its meaning is uncertain (Behind the Name).
Diggory as a surname is not researched. The name is possibly a nod to one of the main characters in C. S. Lewis' Narnia tales, Digory Kirke (Wikipedia). Being a Hufflepuff, the name Diggory may also be a reference to badgers digging holes in the ground. The surname Digger is derived from someone who held the important position of maintaining a dyke or ditch (Internet Surname Database).
Cedric is a fifth-year in PA, which takes place in 1993-1994. In GF, then, he's a sixth year student, but he's old enough to participate in the Triwizard Tournament, just like Angelina Johnson. Like her, his birthday must be in the early fall, between the beginning of school and October 31.
It is quite interesting that Severus Snape explained to Scorpius that Cedric Diggory became a Death Eater due to the humiliation he suffered in losing the Triwizard Tournament. The direct correlation between a humiliating experience and becoming a Dark Wizard is part of Snape's personal history as well, since his "Worst Memory" was about being bullied by James Potter and Sirius Black in front of the whole school, which caused him to lash out at his best friend Lily Evans, calling her a Mudblood and thus losing her forever (OP28). - SIP
From the Web
Writing by J K Rowling on Pottermore: Thestrals
Harry Potter Wiki: Cedric Diggory