Katie and Alicia have always been a pair. Everyone who knows them runs their names together, as if it was unthinkable that they should ever be separated.
For a while, they brought Angelina into their little group, since she was their third Chaser, and Oliver always called them "Katiealiciaandangelina", or just "Chasers". In terms of Quidditch, it's still that way. They're not sure Harry even thinks of them as separate entities, and the twins seem to think that's the natural way to be. They're used to being "Fredandgeorge", after all.
Since the Yule Ball, they don’t see as much of Angelina off the Quidditch pitch. She's part of "Fred and Angelina" now, and in some ways, their trio's back down to two. They don’t mind. After all, they were enough for each other before they met Angelina.
And as only she knows, they're a pair in more ways than one.
Penny and Cho were close friends at school, despite the age difference. It was only a couple of years, after all, and they always got on well. The friendship slipped a little after Penny left: it was almost inevitable, since they couldn’t see each other during term-time.
But when Penny heard what had happened to Cedric, she knew she was needed. Although Cho was only fifteen, her letters to Penny had been full of talk about Ced, and Penny knew it was as close to true love as you get at that age.
So, as if they had never lost any of their closeness, Penny dropped everything to go to Cho's house. When Cho opened the door, she just hugged her friend, and held her as if she'd never let go.