Metamorphmagus
Teddy's being a Metamorphmagus wasn't the biggest secret. Only the whole of Hogwarts and his family knew it. It came of use to him, on numerous occasions such as the time he was pretending to be a confused English schoolgirl for a whole month just to tick off Victoire who was once in a time described so. 'Shut up, Teddy.' 'Oh, but miss, I'm Jane and I'm lost!.' he said in his most fake accent.
And the time some boy broke Vic's heart and Teddy pretended to be the boy's mum and yelled at him for a solid three hours. Not to mention punching his nose afterwards. And when he used to stalk his ex-crush as her best friend ('Teddy, that's creepy.' 'No, Vic. I'm just keeping tabs').
Undoubtedly the best time was when Victoire was mad at Teddy for some reason when she was fourteen, and he turned into a nice old lady and Vic ranted off about Teddy and his adorable thickheadedness for the evening, and Teddy apologising with a card and chocolates afterwards. (Which, by the way, earned him his first kiss.)
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