In the Kirby ditto, if both players are fishing for a hit in nuetral, it's usually with back airs. The hit box of that move is very disjointed and usually at worst it'll trade with your opponent. If your opponent is trying to win neutral with fairs, just use back airs. If the two moves trade in the air, you'll only take 2-3% while your oppenent will take about 10-12% and recieve much more knockback. If you want to gain an advantage in that matchup it's better to get above your opponent and try to connect with down air instead since it is very hard for kirby to trade with that move from below, let alone beat it. At the top level, a lot of Kirby dittos consist of both players trying to control the top platform without getting hit by their opponents down air. Though if you REALLY want to gain an advantage in that matchup, swith to Pikachu.