Oh! He knows about this!! This is a handshake! It's a bit weird to be treated as if he were human, but the friends he's made has really gone a long way towards breaking down the formalities between android and human.

So he reaches and grasps Yuusei's hand in a handshake.

"I'm 9S." He smiles and lets go of the human's hand. "When I arrived here, it was my first time at a carnival, too. The Carnival is full of surprising things, so don't worry about not knowing what to expect. See the sights, enjoy your time here."