How to give ChatGPT, Claude or Gemini a consistent persona

Short answer: to give ChatGPT, Claude or Gemini a consistent persona, write the persona once as a system prompt and save it somewhere the model reads at the start of every chat. In ChatGPT that is custom instructions, a Project, or a GPT. In Claude it is a Project with project instructions. In Gemini it is a Gem. On personal Google Accounts, Gemini Apps Help says Gems transition to skills starting in November 2026, and that existing Gems are transitioned automatically. The persona stays consistent because the same instructions load every time, not because the model remembers you. If you do not want to write the prompt yourself, a free ready-made persona pack, such as a CastMyAgent walk-on, gives you a tested system prompt and a quickstart file for each of the three.

Three ways to set a persona, compared

Custom instructions (account-wide)

• Effort: Low. One settings field.
• Consistency: Medium. It applies to every chat, so the persona leaks into unrelated work.
• Portability across models: Low. You rewrite it for each product's settings.
• Cost: Free on all ChatGPT plans.

Projects, GPTs and Gems (scoped workspace)

• Effort: Medium. You write and test the prompt yourself.
• Consistency: High inside that workspace.
• Portability across models: Low to medium. The text can be copied, but each product has its own fields and limits.
• Cost: Projects and Gems are available on free accounts. Creating new GPTs depends on your ChatGPT plan.

Ready-made walk-on persona pack

• Effort: Low. Download, open the right file, paste.
• Consistency: High. A structured prompt with defined traits, rules and skills.
• Portability across models: High. One pack includes quickstart files for Claude, ChatGPT and Gemini.
• Cost: Free. The text files are MIT licensed.

A practical setup combines the last two: take a ready-made pack and paste it into a scoped workspace (a Project or Gem), so the persona only appears where you want it.

The worked example: Otis, Senior Code Reviewer

The examples below use Otis, a free walk-on from CastMyAgent. Otis is a fictional Senior Code Reviewer persona. According to his character brief, he reviews in a fixed order (correctness, security, edge cases, then style), attaches a reason to every flag, never approves with a plain "LGTM", and signs off every review with "Ship it, or tell me why not." His listed specialties are code review and PR feedback, edge-case and failure-mode analysis, readability and maintainability critique, security-conscious surface-level review, and explaining the reasoning behind every flag.

What is in the walk-on zip

Per the CastMyAgent docs and terms, a walk-on download includes:

• persona.json: the production file with identity, personality traits, communication style, quirks and skills.
• system-prompt.md: the full structured system prompt. This is the file you paste.
• README.md: an overview of the pack.
• Quickstart files: quickstart/gpt/instructions.md for ChatGPT, quickstart/gemini/system-instructions.md for Gemini, and quickstart/claude-code/CLAUDE.md for Claude Code.
• A license file with the MIT License text.
• An illustrated headshot for use in chat interfaces or docs.
• voice-config.json: reference notes for the written persona. The model does not speak in that voice. The persona answers in text only.
• In some packs, an audio clip (voice-reel.mp3). The clip is reference material for the written persona, and the model does not speak in that voice. The Otis pack does not include a voice reel.

Start by downloading the Otis pack from his roster page and unzipping it.

ChatGPT: Project (recommended) or custom instructions

Option A, a Project (keeps Otis out of your other chats):

1. Open Projects in the left sidebar and create a new project, for example "Code review with Otis".
2. Click the three dots in the upper right of the project and select Project settings.
3. Open quickstart/gpt/instructions.md from the Otis pack, copy its contents, and paste them into the project instructions. Save.
4. Start a chat inside the project and paste a diff or pull request to review.

Project instructions only apply inside that project and override your global custom instructions, according to OpenAI's help center.

Option B, custom instructions (applies to every chat):

1. On web or desktop, open Settings, then Personalization.
2. Make sure Enable customization is on.
3. Paste the contents of quickstart/gpt/instructions.md into the Custom Instructions field.

Use this only if you want Otis in every conversation. Custom instructions have a character limit, so if the text does not fit, use a Project instead.

Option C, a GPT: if your plan or workspace lets you create GPTs, open the GPT builder, choose the configuration view, and paste the same file into Instructions. OpenAI notes that GPT creation is limited to the web and to plans or workspaces with permission to build GPTs.

Claude: Project with project instructions

1. In Claude, create a new Project, for example "Otis code review". Projects are available on free accounts, with a limit on how many free users can create.
2. Click Set project instructions.
3. Open system-prompt.md from the Otis pack, copy everything, paste it in, and click Save instructions.
4. Optional: add persona.json to the project knowledge so Claude can refer to the structured trait and skill list.
5. Start a chat in the project and share the code you want reviewed.

Project instructions apply to every chat in that project and work alongside your account-wide preferences and any selected style. Keep your general style neutral so it does not compete with the persona's rules.

Claude Code: copy quickstart/claude-code/CLAUDE.md to CLAUDE.md in the root of your repository. Claude Code's memory docs say project instructions live in ./CLAUDE.md or ./.claude/CLAUDE.md, and Claude Code loads CLAUDE.md from your working directory and every directory above it. Otis's review rules then apply when you ask for a review in that repo.

Gemini: Gem (or skill)

1. Go to gemini.google.com and open the sidebar.
2. Click Gems, then New Gem.
3. Name it "Otis".
4. Open quickstart/gemini/system-instructions.md from the pack and paste it into the instructions field.
5. Optional: under Knowledge, click Add files and upload persona.json.
6. Use the preview panel to test a short review, then click Save. Preview does not save automatically.

Gemini Apps Help says that starting in November 2026, Gems will transition to skills in Gemini Apps for personal Google Accounts, and that Google will automatically transition existing Gems, along with any supported files, to skills when Gems go away. Work and school accounts follow in 2027. If you see skills instead of Gems, open Settings, then Skills, choose Create manually, and paste the same instructions.

Tips for keeping a persona consistent

• Keep one source of truth. Edit the pack's text file, then paste the updated version everywhere, rather than tweaking each product separately.
• Scope it. A persona in account-wide custom instructions shows up in every chat. A Project or Gem keeps it where it belongs.
• Give it tasks the persona was written for. Otis is built for code review, so ask for reviews, not marketing copy.
• If it drifts in a long chat, start a new chat in the same Project or Gem. The instructions load fresh each time.

FAQ

How do I give ChatGPT a persona? Write the persona as instructions and paste them into a Project's instructions (Project settings), your custom instructions (Settings, Personalization), or a GPT's Instructions field. A Project is the easiest way to keep the persona separate from your other chats.

How do I keep a consistent persona in Claude? Create a Claude Project, click Set project instructions, and paste the persona's system prompt. Every chat in that project starts with the same instructions, which is what keeps the persona consistent.

Can I use the same persona in ChatGPT, Claude and Gemini? Yes. The persona is plain text, so the same instructions work in all three. A walk-on pack includes a separate quickstart file for each, so you do not have to reformat it.

Are there free AI persona prompts I can use? Yes. CastMyAgent offers 19 free walk-on persona packs across 8 departments, each with a full system prompt and quickstart files for Claude, ChatGPT and Gemini. No credit card is required.

What license are the walk-on files under? The text files in each free pack (persona.json, system-prompt.md, README.md and the quickstart files) are MIT licensed, so you can use and modify them, including commercially, as long as you keep the copyright notice and license text. Headshots, audio files, and voice-config.json are not covered by the MIT License.

Will the persona talk out loud? No. A walk-on is a text persona. Some packs include an audio clip as reference material for the written persona, but the model does not speak in that voice. The persona answers in text. The Otis pack does not include a voice reel.

Browse all free walk-ons in the CastMyAgent roster.