Lightosia
Smash Journeyman
Okay Pkfire is dumb... If the marth is smart, they get out by upb, downb, or fair (the last two hit you more easily).
If you see they are using down B, just do another pk Fire. When I played a Marth that was using Down B to cancel, I just started spamming PK Fire! (That Marth didn't knew a lot about DI, though)
I dunno if it will work when Marth uses Upb or Fair.