AI character cards: what they are and how to write one

A character card can be as simple or as detailed as you want. Its job is to help the AI roleplay the way you have in mind. There’s no single right way to write one, but a little clarity can go a long way.

What is an AI character card?

An AI character card is a set of details you can save, share, and use in roleplay. It gives the AI information about who or what it’s playing. That might be a person, a creature, a narrator, or something much harder to put a name to.

A card can describe a personality, a whole history, a relationship, or a situation you want to explore. It can be a few lines or a detailed document. What matters is whether it helps the AI understand the role you want it to play.

The card works alongside the AI model, the conversation so far, and any other instructions you’ve added. Different models may interpret the same card differently.

Write your character card your way

You don’t need to follow a checklist or fill out a particular outline. Ordinary paragraphs, notes, dialogue, and detailed biographies can all work. Choose whatever helps you express the idea.

Think about what the AI needs to know to play the role. Depending on your story, that could mean how someone speaks, what they know, how they relate to the player, or how the world around them works. Include the details that matter to you.

If something is important, say it clearly. The AI won’t know the parts of your idea that are still only in your head. If you want it to improvise, leave room for that too.

An opening message can help establish the tone and give the player a place to begin. It can be a question, a scene, a line of dialogue, or whatever fits the experience you’re creating.

Use structure if it helps

Structure is optional. A short card written in plain language may be all you need. As a card gets more detailed, grouping related information can make it easier for both you and the AI to follow.

Markdown

Markdown uses simple headings and lists to organize text. A heading like ## Background or ## Relationships can separate topics without changing how you write the details underneath.

XML

XML uses opening and closing tags to mark where a section begins and ends. You might put background details between <background> and </background>. Choose names that describe the contents and keep matching tags together.

Markdown headings and XML tags are well-established ways to organize prompts. They help make the purpose and boundaries of each section clearer, especially when several kinds of information appear together.

You can use either, combine them, or keep writing in plain text. There’s no required set of headings or tags, and structure alone won’t make a card work well. The information inside still matters most.

Leave room for the story

An AI model can only work with a limited amount of text at once. That limit is called its context window, and it’s measured in tokens, which are small pieces of text.

Your character cards share that space with the conversation history, other characters, lore, and instructions. A very long card can take up room that could otherwise hold more of the story. With several characters, those lengths add up.

This matters especially when running models locally. The usable context depends on the model, your settings, and how much memory your computer has. A model may support a larger window than your setup can comfortably run.

Large cloud models can offer enormous context windows, giving you much more room for detailed cards and long conversations. Some support a million tokens or more. Paid AI services commonly charge for the text they process, so sending more text can also make each reply more expensive. Rates and discounts vary by service.

You don’t have to make every card short. Keep the detail you want, but be aware of what it uses. Removing repetition can save space without changing the character. World details that are only needed sometimes may fit better in a lorebook.

Try it and make changes

The best way to find out whether your card does what you want is to use it. Start a scene, write a few messages, and see how the AI interprets the details.

If something feels wrong, look for information that’s missing, unclear, or easy to misunderstand. You might add a detail, reword an instruction, or remove something that pulls the roleplay in a direction you don’t want.

Try changes in a fresh conversation so earlier replies don’t keep steering the story. If you switch models, you may want to try the card again. The goal is to get the roleplay you enjoy, whatever form your card takes.

For help creating or importing a card in the app, see the character documentation. You can also browse community characters to explore what other people have made.

Advanced: how Museris puts the prompt together

The AI doesn’t receive your character card on its own. For each reply, Museris stitches the selected characters, lore, scene instructions, and conversation history together into the prompt that reaches the model. The system prompt is sent separately.

Your character’s name and definition sit alongside the other characters in the scene, including the character you’re playing. This means the AI reads your card as part of a larger cast and story.

The prompt template controls the layout

The prompt template defines how the prepared character, lore, instruction, and chat blocks are arranged, and what text surrounds them. Museris includes a default template, but you can change it when you want more control over how the AI sees the scene.

Clear boundaries help separate the parts

XML works especially well for marking sections within a larger prompt. Opening and closing tags can make it clearer where character information ends and lore or conversation history begins. Inside a card, they can also distinguish background facts from behavior or relationships.

Think about both levels of structure: how your own card is organized, and how the template wraps the cast and the rest of the story. Clear labels help the model tell which details belong to which role. Keep instructions meant for the whole conversation in the system prompt so they don’t get mixed up with one character’s traits.

You don’t need to edit a template to write a character card. If you want to explore this further, the prompt template guide explains how the pieces fit together.