One of the biggest selling points for Super Smash Bros. Ultimate is arguably the "Everyone is Here!" statement. As you'll likely know, the game features every single playable character that has ever been available in a Smash game - and also throws in a few new ones - but there's one character which actually causes a little bit of a problem here.
That character is Charizard. In Smash Ultimate, Charizard appears as part of Pokémon Trainer, a character originally from Smash Bros. Brawl who can swap between the aforementioned fiery lizard, the ever-adorable Squirtle, and the super-cool grass-type Ivysaur. Technically, though, Charizard has also appeared as a separate standalone character in Super Smash Bros. for Wii U and Super Smash Bros. for Nintendo 3DS (known by fans under the name 'Smash 4').
While we selfishly wait to hear what this means for the Super Smash Bros. amiibo collection (will Charizard technically be included twice?), the official Pokémon UK website has chimed in with a theory as to why this change may have taken place. The paragraphs below come from an article celebrating the franchise's appearance in Ultimate, noting how Smash's Charizard may have disobeyed the Pokémon Trainer for a while, just like Ash Ketchum's did in the Pokémon anime.
The original Super Smash Bros. for Nintendo 64 marked the first time that Pokémon made the jump from turn-based battles to real-time combat, with Pikachu and Jigglypuff as the debut Pokémon contenders. The series has come a long way since then, with a wide assortment of fighters from the Pokémon universe appearing in Super Smash Bros. Ultimate.
The returning Pokémon Trainer can send out Squirtle and Ivysaur, as well as Charizard, which is back at the Pokémon Trainer's side after its previous standalone appearance. We'd like to believe that this Charizard merely decided to disobey its Trainer for a little while, much like Ash's Charizard did in Pokémon the Series.
So there you have it, Charizard just went through its moody teenager phase for Smash 4, before realising the importance of its friends and family for Ultimate. We've all been there, buddy.