Jigglypuff
Sonic: Is this another Pokémon?
Tails: Yes! That's Jigglypuff, the Balloon Pokémon.
Sonic: Balloon Pokémon, huh? I guess I should be able to send it flying then.
Tails: Yes! This fight is in the bag!
Knuckles: Good luck, Sonic! Go beat that thing up for me.
Sonic: Wait, what?
Knuckles: It's just so stupid! All Pokémon have a reason to be here; Pikachu is the mascot, Charizard is one of the most loved Pokémon ever, Lucario is pretty much the poster boy for Mega Evolutions & Greninja is representing the newest generation!
Sonic: So Jigglypuff has no reason to still be here?
Knuckles: YES!
Tails: But Jigglypuff is part of the Original 12!
Knuckles: EXACTLY!
Tails: ...What?
Knuckles: Since Jigglypuff was in the original Super Smash Bros., it can never leave! That's the only thing keeping it here!
Sonic: Yeah, it is quite stupid.
Knuckles: Tell me about it.