Cedric Diggory
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You always knew Cedric was a player, there wasn't a single person on Hogwarts that denied that, but when both of you began dating, you really thought he would change
Oh, how wrong you were
It took it's time, but nearly 6 months into the relationship, your best friend showed you a picture of Cedric and Cho Chang making out before his Quidditch game
As soon as you saw the picture, you stormed towards him, not caring who he was with or where he was