First you must beat the Subspace Emissary mode. Then you must go back to the Forest stage. In this stage you will find the door to fight him (it is near the beginning of the level). Beat him to unlock him.
You don't really unlock him: its more of when you can play as him. If you are Samus by this point in Subspace Emissonary, you will soon meet Pikachu. A cutscene shows where Samus finds Pikachu trapped in a power generator, which shows that the lab is powered by Pikachu's electricity as the main source. Later on, Pikachu and Samus bust out of the room and make their way out of the labratory, which you, the player, must select to be either Samus or Pikachu to battle R.O.Bs and go through other obstacles in order to escape.
you mean pikachu yes
It's not. You can only face them in the Subspace Emissary or Boss Battles.
Finish the subspace emissary.
Beat the subspace emissary.
You can play as him in the Research facility levels, and the Subspace Bomb factory levels.
You . . . don't. There are only two ways to unlock characters in Brawl: either have them join you in The Subspace Emissary--and there are multiple ways to do this, depending on the character--or fulfill the requirements outside of the Subspace Emissary, like completing Classic mode in a certain time limit, and best the character in a one-one brawl. That's it. If you don't want to go through the Subspace Emissary to unlock characters, you'll have to brawl with them. If you don't want to brawl, go through the Subspace Emissary. There is no third option.
Beat Subspace Emissary then he appears
it is in stadium in solo to unlock beat the subspace emissary
complete the subspace emissary you should now be able to play him.
Finish the subspace emissary mode.
Unlock him through the subspace emissary.
Having him join you in subspace emissary is the easiest and fastest way.