She is an American jazz singer who eventually became Black Panther’s fiancé, but their engagement was eventually broken off.
Considering the fact that the film doesn’t actually have any scenes with T’Challa in America, creating that relationship in the Marvel Cinematic Universe probably would have resulted in a very different film than the one we got.
T’Challa first met Monica Lynne in Avengers #73, saving her from an attack by a gang known as the Sons of the Serpent.
Considering the quality of the final product, and the wonderful chemistry between Lupita Nyong’o and Chadwick Boseman, it seems like they ultimately made the proper choice.