Can there truly be any ethical billionaires?

There is the ever increasing discourse that billionaire wealth can only be attained through exploitation. My question is, suppose a millionaire simply kept working and having sky high profits without exploiting workers, could that achieve billionaire status?

Imo, the one and only billionaire who ethically attained their wealth is J.K Rowling (before her TERF crusade). She actually started out poor (she was on the brink of homelessness), wrote the Harry Potter books herself (her own work), and rose to fame and wealth because of her book sales. Writing those books herself did not entail worker exploitation. She is also one of the very few true "from rags to riches" stories.

NOW, after the franchise was over and using her billionaire money to fund her TERF crusade is an entirely different story.

What do you think? Under highly specific circumstances, can billions be made ethically? Or are millionaires the only upper class that can be attained ethically?

(And no, trillionaire status no matter what, cannot be attained ethically)