Though technically not officially confirmed by Marvel, all evidence points to Seo-joon playing Prince Yan, notably Carol Danvers' husband in Marvel comics. With the McDonald's toy line labeling the character as "Prince" and reports saying that is indeed Prince Yan, it is all but confirmed at this point.
Additionally, Livanos previously revealed that Seo-joon would be playing an "ally to Carol," and Prince Yan certainly would fit that bill.
However, especially when compared to other debuting characters like Zawe Ashton's Dar-Benn, very little is actually known about Seo-joon's new character.
New Details About Park Seo-joon's Marvel Debut
In an interview for the newest issue of Total Film magazine, The Marvels director Nia DaCosta and Marvel Studios Director of Production and Development Mary Livanos revealed new information about Park Seo-joon's character, widely assumed to be Prince Yan.
First, DaCosta discussed the various other planets fans can expect to see in the movie — one of which might be Prince Yan's home planet of Aladna, known for its residents speaking only in rhyme.
DaCosta described "bright worlds that you haven't seen before," which could be referring to the world full of dancing inhabitants in bright colors seen in a recent trailer:
"The biggest difference from the other MCU movies to date is that it's ['The Marvels' is] really wacky and silly. The worlds we go to in this movie are worlds unlike others you've seen in the MCU. Bright worlds that you haven't seen before."
DaCosta continued, explaining seemingly of the planet Seo-joon's character is seen on in the movie's trailer that it "is very much high-key, colorful, bright:"
"It was really important to me that every planet felt like a completely different space in terms of colour, lighting, and just energy because if you have the entire universe at your disposal, the planets can't feel the same. That's one that is very much high-key, colorful, bright."
On Seo-joon's character himself - who was shown dancing with Brie Larson's Carol in the latest trailer, DaCosta says that it is "really fun when you realise his relationship to our hero:"
"His character is really awesome, and [it's] really fun when you realise his relationship to our hero."
Livanos, a producer on the film and several other MCU projects, added he is "someone from her [Carol's] past that is meaningful as a friend to her:"
"He's a definite ally to Carol Danvers. Someone from her past that is meaningful as a friend to her, and so he and his people are important to Carol."
Is Park Seo-joon Playing Prince Yan?
With the new information from this interview, it becomes clearer than ever before that Seo-joon is likely playing Prince Yan in The Marvels. He is from a "colorful, bright" planet, has a past connection with Carol, and has "his people," implying him to be a ruler of some kind.
Additionally, there were several rumors of The Marvels being a musical in some capacity, so a rhyming planet where people are seen dancing helps explain where that information may have come from.
Of course, this isn't even mentioning the toy line that all but said it outright.
But for now, all fans can do is wait and speculate until The Marvels premieres in theaters on November 10.