Really, it's the relationship between Tonks and Lupin that suffers in the films, but Remus manages to avoid the list just because he is showcased pretty well in Prisoner of Azkaban. Tonks is a really fun, lovable character, but you only get small glimpses of her eccentric personality throughout the films, understandably due to time restraints. But what we can't forgive is the ham-fisted way their relationship is shoehorned into the movies, where viewers who hadn't read the books probably wouldn't even know that they were together. We sort of hear that they're dating, we sort of hear that they're having a baby, and then BAM - they're dead. While a lot of that subplot - Lupin worrying about their age difference and her throwing her life away on a werewolf, Lupin worrying that his son will inherit his wolfishness - isn't exactly essential, it would have been nice for them to include some information about their son Teddy. The fact that Harry is his godfather, and that Teddy loses both his parents as an infant, creates a nice (if really depressing) sense of symmetry, aligning his character with both Harry and Neville.