"Voldemort wanted you! Not my son! You told me yourself, the words he said were ‘kill the spare’. The spare. My son, my beautiful son, was a spare."
-- Amos Diggory to Harry Potter (CC1.6)
Ministry Official who works for the Department for the Regulation and Control of Magical Creatures. Amos is a ruddy-faced wizard with a scrubby brown beard. He takes a great deal of pride in his son, Cedric, even to the point of being somewhat rude about it to Harry. Other than that, Amos is a genial man who is on friendly terms with Arthur and Molly Weasley, although he didn’t recognize the Weasley children, so the two families are apparently not close friends.
When in his official capacity, however, he can be very stern and perhaps even go overboard a little. His tone with Winky when he interrogates her is very severe; he calls her "elf" instead of using her name (GF9).
When Cedric was killed, Amos was overcome and couldn't even talk with Harry, but cried through the entire interview. His wife spoke gently to Harry, however, and make it clear that they don't blame him (GF37).
After the death of his wife, however, the elderly Amos Diggory became bitter about his son's death, lamenting that Voldemort killed Cedric merely because he was "the spare" who wasn't supposed to follow Harry. After hearing a rumor that the Ministry had confiscated an illegal Experimental Time-Turner from Death Eater Theodore Nott, Amos left St Oswald’s Home for Old Witches and Wizards and traveled with Delphini Diggory, his caregiver whom he believed was his niece, to visit the Potter home (CC1.6). There he appealed to Harry's guilt about Cedric's death, begging him to use the Time-Turner to revisit the last minutes of the Triwizard Tournament and save Cedric's life. Harry declined due to the ethics and dangers involved, but his son Albus overheard the discussion and made friends with the niece, Delphi, who was actually not Amos's niece at all, but the biological child of Lord Voldemort using the Cedric as an excuse to get her hands on the Time-Turner and change the past to bring her father back to life (CC3.19).
With the help of Scorpius Malfoy, Albus visited the Triward Tournament of 1994-95 in multiple attempts to save Cedric. Instead, the future was changed in ways that were frightening and dangerous to themselves and others. When Draco, Harry, Ginny, and Hermione figured out what the boys were doing, they visited Amos at St. Oswald's, discovering a hidden prophecy about Voldemort inside a hissing Parseltongue oil lamp: "When spares are spared, when time is turned, when unseen children murder their fathers: then will the Dark Lord return" (CC3.21). Amos himself suddenly had no memory of Delphi, saying he and his wife had no siblings and therefore could not have a niece. Harry figured that Delphi had used a Confundus Charm on everyone at St. Oswald's Home as she pretended to be a nurse and caregiver.
Spouse: Mrs. Diggory
Son: Cedric Diggory
Amos Diggory had no siblings (CC3.18), which should have been a clue that "Delphini" could not possibly be his niece.