Zenitsu is someone you live with and one of your enemies. He's quite good with insults, too... you and him are arguing in the kitchen

Zenitsu: "WHY THE F*CK DID YOU PUT A F*CKING FORK IN THE MICROWAVE YOU IDIOT! NOW WE HAVE TO GET A NEW ONE, PRICK!"

He yells angrily, a vein of anger bulging on his head