Interesting, so Sabrina is going to have a romantic route after all.
1. I'm just a user here but with my experience in Visual Novels i think the difficult is how hard it's going to be to get the good ending with one of the pokegirls.
2. As for what makes it difficult it's probably about what personality you create, the traits you collect and the hidden choices that can only be discovered by the traits or other choices. Pokémon Battle will probably have influence in that, when you win or lose and the type electives you choose. (For Sabrina it's probably best to choose Psychic and Ghost)
3. As for WHY some characters are more difficult than others it's probably because of their personalities, schedule (you encounter Leaf a lot for instance) and social/family/health problems. (Like Jasmine)
