Eleven

"That's okay," he says. If someone cared about this box, they shouldn't have left it buried on the beach, he thinks, and since they found it, it's theirs to break. Or it's Eleven's to break anyway.

"Bust it up, kid," he encourages. "Let's see what's inside."

Probably something gross, like rotting fish. Hopefully not anything too gross, like rotting human pieces. The thought occurs to Hopper too late, though.