Type effectiveness is a mechanic in Super Smash Bros. Brawl that modifies the amount of knockback taken by Pokémon Trainer's Pokémon from certain kinds of attacks. In Super Smash Bros. Ultimate, Pokémon Trainer returns after his absence in Smash 4, but the type effectiveness mechanic was completely removed. The removal benefits both Ivysaur and Charizard, but negatively affects Squirtle.
Squirtle, Ivysaur, and Charizard each have a weakness and a resistance to certain effects of attacks. When hit by an attack they resist, they take 10% less knockback, while being hit by an attack they are weak to deals 10% more knockback.
While the effect of type effectiveness is minor in Brawl, it is not insignificant. Those who play mirror matches with Pokémon Trainer will try to abuse this mechanic whenever possible, as not only will their main KO attacks be reduced in power when facing their counter, they become more susceptible to their opponent's. It also has a severe effect on matches against other characters: while no other character can use Grass or Water attacks (aside from Kirby's copied Bullet Seed and Water Gun, and Mario's non-damaging F.L.U.D.D.), 13 characters have KO-worthy fire attacks, which overall gives Squirtle more survivability and cuts Ivysaur's noticeably. An opposite benefit can happen with weak multi-hitting attacks such as Flamethrower or PK Fire; Ivysaur's extra knockback allows it to escape the attack easier, while Squirtle will likely stay trapped for longer. Similar behaviour manifests in the Subspace Emissary; no enemies wield Grass attacks and only Hydro Jyks use Water attacks, whereas several enemies and bosses possess flame attacks.
In the Pokémon series, each Pokémon has one or two types, and each attack has one type. Whenever an attack hits a Pokémon, the damage dealt is altered by any weaknesses or resistances caused by the type combination. For example, a Water attack will deal double damage to a Fire or Rock Pokémon, while a Grass attack will deal half damage to a Fire-type, and an Electric attack will have no effect on a Ground-type Pokémon.
The Brawl system is a heavily simplified form of the originally seventeen (Fairy added in games released after Brawl) types and hundreds of type interactions—only three types are involved, and each of the three Pokémon the mechanic applies to has exactly one weakness and one resistance. If all type interactions were carried over into Brawl, Pokémon would take altered damage from several other attack types; such as freezing attacks being effective on Ivysaur. Brawl also only applies any sort of type effectiveness to Pokémon summoned by Pokémon Trainer; other playable Pokémon are not affected in any way.