Against fast characters, I find shorthop nair to be my best friend. It is really quick, can combo be used out of a dash (a lot like an arial dash attack) and you can space it to either be out of range of punish (standing into short-hop nair) or position it so that you land behind them so that you can get away unpunished if they block it. Although it may just be the players I've played against, I've also found success with pivot grab. If they dash at me, I dash into them, and pivot grab. Usually they attacked where I was just a split second ago and I can land a free grab. However, your opponents may be better than that.
Also, if they are shielding, you can shorthop and spam dair on them until they roll away. Not exceptionally useful, but it can help sometimes.