Daphne - Seriously, everything about the character is invented by fans, as the only bits that come from canon are she's a pure-blood and a Slytherin.

We don't know she's actually blonde but she's always portrayed as blonde with blue eyes, we don't know anything about her personality but she's always an "Ice Queen", we don't know who she was friends with yet she's portrayed typically as sympathetic to half-bloods because of a fan-created friendship with Tracey Davis.

Everything about the character is essentially your typical pure-blood OC (aristocratic, knowledgeable about customs, falls for the outsider Harry) yet she escapes the criticism of the label OC because people assume the character is based on something because the name is a Rowling creation.

Susan - Though she appears more (barely) in canon, Susan Bones is nearly as successful in that regard. She is the generic Good-Ministry-Pairing for many stories. Think about how fanon it has become regarding her assets shall we say. She is the amazing, curvy redhead bombshell with the biggest bazookas in the school.

Hadrian - Lord Hadrian Black-Potter-Peverell-Gryffindor-Slytherin-Merlin-LeFay of the most ancient and noble of the House of Potter who is heir to more than one ancient family or is a direct descendant of Merlin whose title comes with boons such as having to have a wife for each family title and everyone would having to do what he says.

In general, Hadrian has become a fanon shorthand for "Pureblood-wanking heir of the Most Ancient and Noble House of Potter-Black-Slytherin-Gryffindor-Ravenclaw-Hufflepuff-Ambrosius-LeFay-Christ of Nazareth-Pendragon-Peverell-Slytherin-again-for-good-measure, who is constantly being harassed by Dumbledore and the Weasleys so he can knock up Ginny and hand them his inheritance before croaking it for the Greater Good, but who has the power of God and aristocracy and remembering goblin names on his side.

 

Do we even know that Daphne is a pureblood from the books?

 

We don't even know she's Slytherin from the books, she's literally just a name passing OWLs in 5th year. Then JK mentioned her when saying Malfoy married the sister of a member of Pansy's gang. She might have been mentioned as a Pureblood Slytherin before that, on her list of 40 students, but she was called Queenie and plenty of things changed from that list.