Harry Potter fans are fuming over the latest character reveal. And for good reason.

With release of the Fantastic Beasts: The Crimes of Grindelwald trailer, a long-held fan theory from the Harry Potter universe was confirmed. Nagini, Lord Voldemort’s serpentine sidekick, was once a witch.

(If were going to be technical about it, she’s a Maledictus – ie. she carries a blood curse, passed from mother to daughter, that turns a human into a beast… Which is almost certainly the nerdiest sentence ever written. But moving right along.)

Yet after the crows of “we knew it!” died down, some fans came to a rather disappointing realisation about the casting of the snake’s humanoid form.

The trailer revealed that Nagini is played by South Korean actor Claudia Kim, which some are arguing is problematic and even potentially racist.

See, when rumours first surfaced that the 33-year-old would have a role in the J.K. Rowling-scripted sequel, it was hailed as a major boon for the franchise and a step-forward for representation of Asian actors in Hollywood.

But now that we know she is essentially playing a white man’s pet… well…

In the film, set roughly 70 years prior to the Harry Potter series, Nagini is a star attraction in a wizarding circus/freak show, in which a sinister ringmaster exploits her transformative powers for entertainment.

Those familiar with the Harry Potter books/movies will know that she is destined to become the loyal pet of the series' arch villain, Lord Voldemort. So loyal, in fact, that she plays host to part of his soul, which makes her a target of the good guys trying to destroy him. [Spoiler: in the end, she's killed = he's killed.]

"This feels like a problem," young adult author, Ellen Oh, wrote on Twitter of Kim's casting. "I feel like this is the problem when white people want to diversify and don't actually ask POC how to do so.

"They don't make the connection between making Nagini an Asian woman who later on is the pet of a white man. So I'm going to say it right now. That shit is racist."

And she wasn't alone in her concerns. Backlash pointed to tired TV/movie tropes of Asian women as exotic, fetishised, enslaved.

“Let's retroactively turn a woman of color into a literal object owned by a white man inspired by nazis.” https://t.co/JYTQK6YFss

— Bad Janet™️ (@CharlesPulliam) September 25, 2018

maybe “circus performer who turns into a murderous snake“ isn't the best choice of role for one of the two WOC in the franchise.

— Gavia Baker-Whitelaw (@Hello_Tailor) September 25, 2018

A problem I have w/ the after-the-fact diversity tack-ons is that these characters & ANIMALS were written as NOT POC. But now that they're being made POC their behavior doesn't sync up. Like Wormtail MILKED Nagini to feed gross baby Voldemort in HP4 and now she's an Asian woman?!

— Kat Cho ???? (@KatCho) September 25, 2018

Screenwriter and creator of the Harry Potter universe, J.K. Rowling, has now responded to the casting criticism.

One Twitter user wrote to the author, denouncing the casting choice.

"Listen Joanne, we get it, you didn't include enough representation when you wrote the books," Jen Moulton wrote.

"But suddenly making Nagini into a Korean woman is garbage. Representation as an afterthought for more woke points is not good presentation.”

Just an hour later, Rowling replied.

The Naga are snake-like mythical creatures of Indonesian mythology, hence the name ‘Nagini.’ They are sometimes depicted as winged, sometimes as half-human, half-snake. Indonesia comprises a few hundred ethnic groups, including Javanese, Chinese and Betawi. Have a lovely day ????

— J.K. Rowling (@jk_rowling) September 26, 2018

Actress Claudia Kim hasn't responded to the criticism either. The actor has so far expressed nothing but enthusiasm about her role.

“You’ve only seen her as a Horcrux. In this, she’s a wonderful and vulnerable woman who wants to live,” she told Entertainment Weekly.

“She wants to stay a human being and I think that’s a wonderful contrast to the character.”

The film's Australian release date is November 15.