Order of the Phoenix

Michael Corner attends the first meeting of Dumbledore's Army (although that name hasn't yet been chosen) at the Hog's Head in Hogsmeade. After the meeting breaks up, Hermione tells Harry and Ron that Michael had been there because Ginny Weasley had attended. Ginny and Michael, she says, are currently going out together. This causes Ron to state that he doesn't trust Michael. Carefully, because she was aware that Harry and Ginny had been speculatively linked, Hermione goes on to say that Ginny had been waiting for Harry but had given up, and had met Michael at the Yule Ball in the previous year. They had started going out shortly after that.

As Harry is unable to play as Seeker on the Gryffindor Quidditch team, Ginny takes his place. The final match of the year pits Gryffindor against Ravenclaw; and when Ginny catches the Snitch out from under Cho Chang's nose, Michael sulks instead of congratulating his girlfriend. Ginny then "ditched him, and he went off to comfort Cho Chang instead." On the Hogwarts Express, when Ginny tells this to Ron and Harry, Ron hints that she might now be able to find someone better, implying that Harry would be that someone; Ginny says that she's going out now with Dean Thomas.

Half-Blood Prince

Michael Corner asks a question of Professor Slughorn during his and Harry's first potion class.

Deathly Hallows

As he leads Harry, Ron, and Hermione into the school from the Hog's Head, Neville mentions that Michael Corner had gotten caught releasing a first-year student that the Carrows had chained up as punishment. Michael had gotten very badly beaten for that, which had slowed down the activities of the revived Dumbledore's Army; Neville, the one remaining leader, felt that they could not ask anyone else to go through what Michael had suffered.

When Harry returns to the school, Michael Corner is one of the students in the Room of Requirement waiting for his return. He does have a role in the ensuing battle, but we don't see much of him.