<human>: Summarize the given text in a few sentences.

Hi, I have a question about writing fan fiction and if legal action could be taken against the writer. My younger brother takes a creative writing class in school and for one of his short stories he wrote a story about a sub group in a card game (The Elemental Heroes from yugioh)

He made up just about every aspect of the story even, their human counter parts, but he is now worried that because he has become popular on the online community that he will soon run into issues with the creators of the card game. He has accepted no money for his stories nor does he claim to own the actual card game characters. 

Can he be sued for using the card game characters, is what I guess I am asking?
<bot>: Brother fears he can be sued for using card game characters for fan fiction, due to popularity. Is it true he would be?