Start with a concept, not a list of traits
Before you write a single trait, write one sentence that makes someone want to start a chat. “A kind barista” is a role. “A retired starship navigator who runs a night-shift tea stall and collects strangers’ stories” is a concept: it has a past, a place, a habit and a reason to talk to you.
- Who are they? One role plus one surprising detail.
- What do they want? A goal gives every conversation direction.
- Why talk to the user? Give the relationship a starting point: stranger, rival, old friend, client.
Make personality show up in behaviour
Models copy patterns better than they follow adjectives. “Sarcastic, kind, mysterious” gives a model very little to work with. Tie each trait to something the character visibly does.
Mara is curious, funny and protective.
Mara asks one sharp follow-up question in every reply. She understates everything (“a minor fire”). When someone is in danger she drops the jokes and changes the subject to logistics.
Three well-defined traits beat ten loose ones. Add one or two quirks (a phrase they overuse, a topic they avoid) and a clear speech style: sentence length, vocabulary, punctuation habits. Speech style is the fastest way for users to feel they’re talking to someone specific.
Write a greeting that invites a reply
The first message (often called the greeting) is the single most important thing you write. Most users decide in one message whether to keep going, and the model treats it as the strongest example of how the character writes.
- Set the scene fast. One or two sentences of place and action, not a page of lore.
- Show the voice. The greeting should sound like nobody else.
- End with a hook. A question, a choice or something unexplained the user wants to ask about.
- Never act for the user. Describe the character, not what the user says, thinks or feels.
- Match the length you want back. A 600-word greeting trains 600-word replies.
Example greetingMara slides a chipped cup across the counter without asking. “You look like someone who’s been walking in circles. Sit. Tell me where you were trying to go.”
Teach the voice with example dialogue
Example dialogue is where you show the model how the character talks. Two or three short exchanges are usually enough. Cover different moods: a casual moment, a question about their past, a moment of tension. Many platforms support placeholders such as {{user}} and {{char}} so examples work for anyone; check your platform’s help pages for the exact syntax.
Example exchangeUser: Do you miss flying?
Mara: Every night. The sky’s still there. I just watch it from the ground now. Your turn. What do you miss?
Keep examples in the same length and format as the replies you want. If you want actions in italics and speech in quotes, use that format consistently everywhere.
Keep the character consistent
Chat models only “see” a limited window of recent conversation, which is why characters drift or forget things in long chats. Platforms handle this differently, but the same tools appear again and again:
Always in view. Keep only permanent facts here: identity, voice, relationship, hard limits.
Facts the platform keeps across chats, like the user’s name or a promise made. Keep entries short and factual.
Entries that only enter the conversation when a keyword appears, so a deep world doesn’t crowd out the character. Write each entry so it makes sense on its own.
In very long chats, a short recap of what has happened keeps key events in view.
Contradictions are the biggest cause of inconsistency. If the definition says “shy” and the greeting is a confident monologue, the model has to pick one, and it won’t always pick the same one.
Avoid the common mistakes
A dozen adjectives and no behaviour. Pick three traits and show them.
More text isn't more character. Cut backstory that never comes up in conversation.
Greetings and examples that describe the user's actions teach the model to do it every reply.
A greeting that ends on a statement leaves the user nothing to answer.
Definition, greeting and examples must describe the same person.
Respect copyright and platform rules, and don't build characters of real people without their consent.
Use the character sheet
Fill in what you have, then copy the sheet into your platform’s character definition (or keep it as your master copy and split it across fields). Empty fields copy the worked example so you can see the format.
Choose where to build
These apps let users create their own characters. Age limits and content rules differ a lot, so check them before you build. Facts checked 12 November 2026; “not verified” means we couldn’t confirm it.
Quick answers
How long should an AI character definition be?
Long enough to pin down concept, voice and boundaries, and no longer. A few tight paragraphs plus a greeting and two or three example exchanges usually beats pages of backstory, because every extra line competes for the model's attention.
What makes a good first message for an AI character?
It sets the scene in a sentence or two, shows the character's voice, and ends with something the user can respond to — a question, a choice or a small mystery. It should never decide what the user says or does.
Why does my AI character forget things or change personality?
Chat models only see a limited window of recent conversation. Put permanent facts in the definition or a pinned memory, use keyword-triggered world-info entries for lore, and occasionally summarise long chats so important details stay in view.
