And yeah, Lillie here is fairly accurate to the game. Well, the first ones at least. The Ultra versions change stuff up so much it's hard to say. I quit playing when I got to a certain area and an important scene was entirely replaced.
If you quit because of that change, could you elaborate in a spoiler?
It's the scene on Exeggutor Island, in the rain. A really touching scene between your character and Lillie. In Ultra Lillie doesn't even go to the island with you. I was a bit annoyed by some other story changes, mostly Lusamine being altered to make her a bit less of a crazy abusive mom and more like she's just obsessed with saving pokemon from the story's big-bad, but that was the bit that finally turned me away.
For what it's worth, I really liked a lot of the gameplay changes in Ultra, particularly all of the new interactions with random NPC trainers. Add that to the story from the originals and I'd be a happy nerd.