I meant to say Guest Star, which would also include the Dream Friends. Piranha Plant's inclusion means generic enemies can make the list and Star Allies' Guest Star mode is playing as the enemies.
What about Tails using his mech from SA2? Similar to Bowser Jr. in his clown car (not an echo).
Mallow. People are fixing their attention on Geno, but I'm wondering about Mallow.
A dream friend from Kirby Star Allies. Would any of them work? Other than Bandana Waddle Dee since he's the most...
Notice how one of Grubba's moves involve boosting his attack and defense, then when you get the gold star, you get to boost your own attack and defense. Clever little detail.
Because Sakurai created both Kirby and Smash and since the Kirby characters are his, he's more familiar with them than the other characters.
Pit was watching the battle between Mario and Kirby.