Send ✄ for a PET PEEVE headcanon.
Pieter is normally very calm and hard to anger. That’s because he has a tendency, when he does lose his grip on it, to be rash and rather explosive. The quickest way to make him angry? Insult his friends. Be a generally despicable person. And Arceus help you if you hurt his family, be it blood-related, friendship-related, or ANY of his Pokémon.