Flowey is the main antagonist of Undertale. He's not necessarily the most powerful without a booster, so he focuses much more on being threatening and deceptive. Multiple things are needed for this voice: While the voice at times should come off as friendly and up-beat, there can be times where it will switch to intimidating and malicious. At times, you'll also need to be able to talk in the patented sickeningly sweet tone you get from someone who's either making fun of you or really doesn't feel like being the good guy. It's also important to note that, while it is of course not required, most takes are in a higher register. If you can pull these emotions through and pull the pitch up, then good on you!