Consistent with chosen one

To clarify, did you want to be able to create a background image, then use the same background for multiple images with new contact like a character in different poses?

You could use Photoshop and then add AI content through Generative Fill. If you wanted the same person added but say in a different pose, that can be tricky because even if you have the same description with a slight pose variation in the prompt, the character can look different everytime you ask Firefly to generate another version.