Marcus Flint - The only one of mine that Percy's actually probably spoken to, what with school and all. That said he doesn't like him, at all. Why? Because he's a twat and he doesn't like most people, even the ones he's sleeping with. He's the captain of the Falcon, and on the England team, and he says captain of that too. He might talk to Percy,a nd probably be rude.
Theodore Nott - He's 4 years younger, so I don't imagine much Hogwarts interaction. He does work for the Ministry though, as a Hit wizard, so they might have run across each other there, to some extent. Theodore will possibly be polite, to cold, depending on his mood.
Mandy Brocklehurst - Also younger, but works in St. Mungo's. She's involved in the Order, which I doubt Percy would know, and I can't see them interacting much on the work front, but she might reply to his entries, and would certainly be nice.
Megan Jones - Muggleborn, and doesn't work in the Ministry, currently studying Interior Design at a muggle school and interning in a shop in Diagon.I don't think she'd know him either, but as a Weasley, Megan will be inclined to chatter at him.
Gwendolyn Montgomery - Actress, and 5 years younger, so I can't imagine either remembers the other from Hogwarts. Gwen doesn't ever go to the Ministry, so no work interaction either. She might bug him via the journals, especially if he says something interesting, but other than that I think it'd be randoms.