Kirby: 55-45
Zelda and Wolf: 35-65
As for Kirby, I find that he is very nimble in the air, but so is ROB. In the air ROB beats Kirby, but not by much. On the ground, I'd say it's also in ROB's favor, but not by much again, however I just find that racks up damage moreso than ROB does.
Zelda: From what I've seen and played, Zelda ***** ROB. Zelda has some problems racking the damage, but she has so many kill moves that I find it so hard to beat Zeldas. And because ROB is so big, her Fair and Bair hit much easier than usual.
As for Wolf: I find it insanely hard to approach him. "You don't approach, he does." That's not the case. You can't camp Wolf, he can literally just reflector putting your lazer and gyro out of business, and on the ground, Wolf completely ***** ROB and his Bair is so good at approaching, it's hard to beat it.
These are short summaries. I'm tired and want to go to bed.
e: I don't find GaW too hard actually, they get predictable with approaches.