Well, it can't be Harry and it can't be You-Know-Who - we need them around for Book 7.
I doubt it'll be Ron. When he looked into the Mirror of Erised in Book 1, he saw himself as Head Boy and Quidditch captain. The grounds for his achieving these positions (being named a prefect and making the Quidditch team, respectively) were laid in Book 5. So I suspect he'll survive Book 6 and fulfill his desire as seen in the mirror. (Besides, he still hasn't hooked up with Hermione.
)
I hope it isn't Hermione!
She's a supporting character, but a pretty major one.
I think it will be a minor supporting character, but one of the "good guys" - not Malfoy or his henchmen, Malfoy senior, any of the Dursleys, etc. One of the other Weasleys perhaps?
Slight hijack: It's been a while since I've read Book 5. Is Harry's Quidditch ban still in effect as of the end of the book? Quidditch just isn't Quidditch without Harry on the team...