From SmashWiki, the Super Smash Bros. wiki
The Ruins is a side-scrolling maze that has many obstacles, secrets, and Subspace enemies. Ivysaur is found as a trophy and captured while Charizard has to be defeated to be captured for the Pokémon Trainer's use.
After the Ruins are cleared and the Subspace Emissary is completed, one can play through this level again to find a secret door that leads to a battle with Wolf. Defeating Wolf rewards the player with him joining your party and unlocking him (unless he has already been unlocked by alternative means beforehand).
This stage contains the most places where a character can be crushed.