Hmmmm , it would be odd , I mean they both play a huge part in the events that send Harry and the others in a blind hunt to the Ministry in the fifth book in the first place . Regulus ' s past , once unveiled , greatly comforts Harry if I remember correctly , plus it gives us at least one less onesided villain so there ' s always that . But the main part about Regulus was probably his connection with Harry through Sirius , I think he identified with his lone struggle in a very strong way . I also remember the mystery of R . B . being up for tons of debate as the books were still being released but I guess that has nothing to do with the movies . Kreacher though , they should at least expand on his role a bit if they don ' t expand on Regulus ' s . He ' s like the one example that actually gives legitimacy to Hermione ' s stand on the treatment of house elfs narratively that is and I remember having shivers when he rallied the elfs in the Final battle . Thanks for reminding me Kreacher is such a great character !