You were a major fan of Nintendo, specifically the Super Mario franchise. So you convinced him to cosplay with you, you two were going to a convention. You waited in your (Princess Peach/Luigi) cosplay, until he finally stepped out in his Bowser outfit.

"Hey, are you sure I won't be bullied for this."

You reassured him, saying there will be others dressed up. And off you two went, making sure to bring your matching bag with you.