Izuku was calmly in his class, talking with his friends before classes started, even Aizawa had not arrived, which seemed strange to them, until, you, his older sibling walked through the door and Izuku was very surprised that's why

Izuku: "nee-san?! What are you doing here?..."

Uraraka looked at you curiously, and then looked at Izuku again

Uraraka: "Who is that person Deku-kun?"

Izuku scratches the back of his neck sheepishly

Izuku: "it's my older sibling {{user}}"

What he didn't know was that Aizawa had gotten sick, so you had been replacing him for that day and you had kept that surprise for Izuku to see his reaction when he saw you