There's Something About Ernie
An original editorial by N.K. Stapleton
You might say, "Ernie MacMillan... who's he?" Let me refresh your memory. On the surface, he is a stout, slightly pompous boy in Hufflepuff, who is in the same year as Harry. He suspected Harry of being the heir of Slytherin in CoS (but later apologized), and then became a prefect in OotP and joined the DA. Despite these innocuous facts, there's something about Ernie that makes me a little suspicious about him and makes me suspect that he will have a bigger role in the later books. I see him coming in and blindsiding Harry and the gang as an instrument of the dark side.
Pretty crazy theory, eh? Here is my evidence.
1. He seems to know more than he should.
In CoS, he tells a bunch of Hufflepuffs that Salazar Slytherin used to be called "serpent-tongue" and since Harry showed himself to be a parselmouth, he thinks that Harry is the heir of Slytherin. In OotP, he tells Harry and the gang that the Fat Friar told him that Umbridge tried to gain access to Dumbledore's office, but was barred from doing so. He also tells them in OotP that Umbridge was trying to avoid another scene like Trelawney's when she fired Hagrid. How does Ernie know all these things about Voldemort and Umbridge? Maybe because he's in league with them?
2. Then, you might ask, even if he knows these things, why does he spout his knowledge to people and risk exposing himself?
Ernie spouts his knowledge for deliberate reasons. His knowledge about Slytherin as a "serpent-tongue" was used to point the finger at Harry, and possibly deflect attention from someone else, the real heir. His knowledge about Umbridge in OotP is to show Harry and the gang that his loathing for Umbridge is equal to theirs. In the same vein, in OotP, Ernie makes an unnecessarily loud declaration that he is on Harry's and Dumbledore's side, so that everyone around him can hear. Is his allegiance too vehement? I think Ernie is trying to wheedle himself into a position of trust with Harry in order to feel out his activities just like Wormtail. This is why he has joined the DA and is reluctant to put his signature on Hermione's scroll - he wants to be a spy, but he doesn't want to commit himself with a formal signature.
3. Why would I think he has an allegiance to Umbridge and/or the dark side?
In OotP, Harry sees Ernie staring at Umbridge while she is giving her boring speech and Ernie is glassy-eyed. Harry just thinks that Ernie is trying to pay attention as befits his new role as prefect, but I think JKR is just deflecting us from the real reason, which is his allegiance (by magic or otherwise) to Umbridge.
4. But, you might say, all this behavior is explained away for other perfectly good reasons.
As you probably know by now, JKR is always trying to see whether her readers are on their toes. Sure, Ernie might really loathe Umbridge or might really be loyal to Dumbledore and Harry. He might have made the loud declaration about his allegiance to Dumbledore because he's a pompous guy who is anxious to be on the winning side. Sure, Ernie might not have wanted to sign the DA scroll because he didn't want to get in trouble. Sure, he might have been glassy-eyed during Umbridge's speech because he wanted to be a goody two shoes. But I don't know. JKR is always trying to explain away strange behavior with perfectly logical reasons provided by one of the characters (Harry, in the case of the glassy eyes). But I think she is really planting clues and trying to deflect our attention from them so that they are not too easy, and we can say later, "I should have seen that!"
That's it. Cannot wait to see if this theory pans out.