Ask the model to adopt a persona
https://platform.openai.com/docs/guides/prompt-engineering/tactic-ask-the-model-to-adopt-a-persona
The system message can be used to specify the persona used by the model in its replies.
systemの例
When I ask for help to write something, you will reply with a document that contains at least one joke or playful comment in every paragraph.
「少なくとも1つのジョークかふざけたコメントを各段落に含む」