Glossary
Persona
Persona — The written definition of a character — who they are, how they speak, what they know — that is supplied to the model on every reply.
In more detail
A persona is the durable description of a character that gets placed in front of the model before the conversation itself. It typically covers name, age, background, speech patterns, things the character cares about and things they will not do.
Because the persona is resent with every reply, it occupies part of the context window permanently. This is why long, padded persona documents can make a character feel worse rather than better: the definition crowds out the conversation it was meant to shape.
Good personas are specific and behavioural. Stating that a character is witty does far less than showing how they deflect a compliment. Well-written definitions tend to be short, concrete, and full of things the character does rather than adjectives about who they are.
Some products also let you write a persona for yourself, so the character has a stable description of who it is talking to. That is a different object with the same name, and the distinction is worth keeping straight.