The more characters you know,the better your skills get,also,with more character knowledge you could find interesting ways to tackle a match-up,but if you haven't mastered your main to full extent,do that first.
I mean I've been using Momiji alot. Though trying to figure out the MU by just using the
AI and by playing against my cousin don't help much. AI cheats so its kinda pointless, and my cousin, although he's a real opponent, I almost might as well consider him an AI on an online opponent.
I personally think the AI plays "better" than your cousin. And the AI doesn't cheat as much as you think when you understand how it operates (though it still does some sketch stuff like WUK's off of slump statuses).