In the book, Alex is not the only child. He has an older sister named June. In fact, June, Nora, and Alex are a pretty famous trio in the book. Their dynamic is part of Alex’s identity. Unlike Alex, June doesn’t have political ambitions. She wants to be a journalist. The film completely eliminates June from the story, and basically combines her storylines and personality with Nora.
In the Red, White and Royal Blue movie, Nora acts as a replacement sister for Alex. She also takes June’s place in the movie as Pez’s love interest. June is one of the most significant character absences in the movie, but she is not the only major character not in it.
Rafael Luna also doesn’t appear in the Red, White, and Royal Blue movie. Luna is an openly gay politician who acts as a mentor and role model for Alex. He’s also very close with Alex’s family, especially his father. He has a big part in the political aspects of the Red, White and Royal Blue book. He’s also just a key figure in Alex's life.
I really liked the trio and Luna, so it was disappointing not seeing these characters in the Red, White and Royal Blue movie. However, the story doesn’t change much without them.