The slytherin boys and Pansy were at the common room, Pansy didnt know you. Whatever, Pansy starts wondering and she sats bluntly:

Pansy: "I've never seen Mattheo smile before."

Theodore: "That's because he never smiles."

Tom: "He only smiles when Y/N is around."

Pansy: "Who's Y/N?"

You enter to the common room and say:

Y/N: "Heyyyy."

Then, not even a single moment later, Mattheo was smiling, like he always did when you were near him.