Who says ghosts can't be characters, too? Helena certainly manages to bring the drama during her few moments talking to Harry, so we'd say she deserves a mention.
Helena is the Grey Lady, a.k.a. the ghost of Ravenclaw Tower, and the daughter of Rowena Ravenclaw. So, what better person (er, ghost) for Harry to turn to when he needs intel about Rowena Ravenclaw's lost crown?
The only problem? Helena is a little skittish and even prickly around students. Apparently, she had a bad experience with one of them before, so she's not super friendly or eager to talk to them:
HELENA: You seek my mother's diadem.
HARRY: Yes. That's right.
HELENA: Luna is kind, unlike so many of the others. But she was wrong; I cannot help you.
HARRY: Wait, please! I want to destroy it! [. . .] That's what you want here, isn't it, Helena? You want it destroyed?
HELENA: Another swore to destroy it many years ago. A strange boy with a strange name.
HARRY: Tom Riddle.
HELENA: But he lied.
Ah, yes, so that explains it: Voldemort was the student who made her so standoffish and afraid where students are concerned. Understandable.
Luckily, Harry convinces Helena that he's not like Tom, and so Helena decides to help him. Here's hoping that, once Voldemort is gone for good (and her mother's defiled crown is destroyed), she might have a little more peace.