The phrase "underdeveloped characters" refers to characters in a book, movie, or play who are not fully explored or have limited depth. They may lack interesting qualities, complexity, or growth throughout the story. Full definition
Instead it's merely another slack thriller with underdeveloped characters and sputtering dramatic momentum. (hollywoodreporter.com)
True Story not only underutilized it's talented cast, but also made the mistake of underdeveloping the characters. (theyoungfolks.com)
Once Upon A Time In Venice is clearly a film made by cinephiles, but the scattered subplots and underdeveloped characters combine for an overall misfire. (rottentomatoes.com)