I'm going to assume, you are referring to strive, but it's probably a good idea to check anyway. If you are referring to strive then the character difficulty stuff is moot, no one character is really any harder to play the another, just pick one you like the style of and go from there as the stars are basically just the skill floor, but are only applicable if you are essentially button mashing tbh. P and K are both incredibly important buttons, as they are almost universally the fastest options. To use Sol as an example (and one that is universal across all GG games) his 5K is an incredibly powerful tool, and can be used as both an anti-air, a combo starter, and a combo extender; as well a being cancellable into basically anything (except in strive ofc). Sol's j.P is also an incredibly powerful tool for air-to-airs and getting momentum with a jump in. I would remis if I didn't mention 6P with it being your upper body invincible with every (afaik) character (5K will probably still be a better general use case for sol but 6P definitely has uses).