Barty closed his journal after he saw Evan close his. Evan was the only person he had been writing with. But then the comment made him laugh. And Barty chuckled, honestly did. He was doing magic for Evan, and Evan didn't believe it existed. Evan - pureblood, death eater, wizard.
Standing up, Barty held up his wand and decided to do it silently. Concentrating, as he knew Evan was watching, Barty cast the spell and made himself invisible. Looking down he smiled, oh yes. "Can you see me?" he asked cheekily. His voice confirmed he was still there. It was utterly ridiculous and silly, but luckily for Evan those were things Barty liked.