I'm not sure what the unlock conditions are for the characters either. All I did yesterday was a bunch of matches against the AI, and I unlocked Ness, Wario, Falco, and Lucina. I think for Lucina though you just have to battle on the Arena Ferox stage 5 times or something; she appeared after I finished a battle on there. And yeah, they do seem harder this time around, which is strange. I remember them being kinda easy in the past. I lost to Lucina the first time I blame counter spam