1. I don't mean reoccuring characters in the series. I mean characters unique to one game. Like Mallow and Geno are unique to and would represent SMRPG. SMRPG, while great, only really needs one playable character. And that would be Geno. And they would only work together as a duo if their attacks were the same, which they're very much not. They don't even have the same weapons.
2. Yes, but Meta Knight is a reoccuring character, not to mention insanely important to the plot of the majority of the Kirby games. Mallow was, in the end, moderately important to the plot and, unlike Geno, was never seen or heard from again after SMRPG.
NOTE: I DO NOT HATE MALLOW! I just don't think he has a chance of being playable. He'd make a great AT, though.