Hi,Anja Mueller
Welcome to the Microsoft Community and thank you for sharing your issue. I have understood your problem.
I understand you are looking for a way to have Designer retain character features to create a series. Unfortunately, this feature is currently unavailable. However, you can provide feedback to the Microsoft Designer team to request the addition of a feature to save characters for future prompts. The feedback option is located at the bottom of the Copilot Designer AI page.
Although I am not a member of the Microsoft Designer team and my response may not be professional, I can only offer some basic ideas for your reference.

1.Clarify Character Design Specifications
You can refer to the settings in the image and then determine the basic attributes:
Determine the core characteristics of the character (e.g., hairstyle, color scheme, clothing style).
Record: Color and font specifications (for matching text).
Example:
Main color: (Orange), Secondary color: (Purple).
Utilize Microsoft Designer's Template Function
Steps2:
Open Microsoft Designer, select "Create with AI" → "Start from Template" or use a preset template.
After designing the first character, click the top right "..." → "Save as Template", name it "Character Base Template".
For subsequent designs, directly use this template to ensure consistent proportions and framework.
Create Reusable Design Components
3.Save character elements:
Split the character's head, clothing, accessories, etc., into separate elements and export them as PNG (with transparent background).
In Designer, click "Upload" → Add these elements to "My Resources" for easy drag-and-drop use.
After creating the above content, you can use layers, add pages, and duplicate pages to create some simple characters.
I hope the information I have provided can help you, if you need to carry out more professional operation and function then I suggest you still to Designer team for feedback, thank you for your letter
Allen L- MSFT | Microsoft Community Support Specialist