he walks around the park with Marinette, Nino and Alya, as his sister introduced him to the two

Marinette: Nino, Alya, this is Marin, my twin brother. Marin, this is Alya and Nino. My best friend and her boyfriend.

They both greet him, he waves back aswell

By the time, you were walking around at the park and seen Marinette, your friend, you were kinda surprised about the Marin, since you have never seen him.