Philosopher's Stone

When Lord Voldemort is defeated, there is mention on the Muggle news that someone seems to be setting off fireworks in Kent for no good reason. Professor McGonagall mentions that she thinks it is Dedalus Diggle doing it; she says he never had much sense.

Before he is identified to us, on the day after Voldemort's initial defeat, he apparently stops Vernon Dursley to invite him to celebrate. Diggle is later seen bowing to Harry in a store, although Harry does not recognize him.

The first time we actually see Dedalus Diggle and have a name given for him is in the Leaky Cauldron, when Hagrid introduces Harry to the witches and wizards there. Diggle is overjoyed that Harry remembers him, dropping his top hat in his excitement.

Order of the Phoenix

Dedalus Diggle is a member of the Advance Guard, the group of wizards that Alastor Moody brings to Privet Drive to bring Harry to the Headquarters of the Order. Other members of the Advance Guard are Nymphadora Tonks, Hestia Jones, Sturgis Podmore, Emmeline Vance, Kingsley Shacklebolt, Elphias Doge, and Remus Lupin. Dedalus Diggle, as usual, is extremely pleased when Lupin introduces him again to Harry, dropping his top hat in his excitement.

Deathly Hallows

Dedalus Diggle is one of the two Order wizards who are detailed to accompany Vernon, Petunia, and Dudley Dursley to a place of safety four days before Harry's seventeenth birthday, when the protection afforded Harry will cease. Dedalus does not exactly inspire confidence in Vernon by his admission that driving a car is far too confusing for him. Additionally, all of the Dursleys find it alarming that Dedalus' watch chastises him loudly for being late.

Speaking with Harry at Grimmauld Place, Lupin mentions that in the Death Eaters' search for Harry after the fall of the Ministry, Dedalus Diggle's house was burned down, but Dedalus was not there.