Complete one of the objectives for each character, not all 3 are necessary. All characters can also be unlocked in the Subspace Emmisary and Brawl Mode. If you unlock them in Brawl mode the winner fights the secret character.
Play 130 matches in Brawl mode, then defeat him in battle.
Play 15 matches on Shadow Moses Island stage, then defeat him in battle.
Complete Stage 23 of the Subspace Emissary and Solid Snake will join your party.
The best way to unlock Snake is to probably just play 15 Brawl matches on Shadow Moses Island. All you really have to do is set the computers on Lv. 1, 1 stock, and just smash attack the life outta them. X3
Play 160 matches in Brawl mode, then defeat him in battle.
Collect 250 trophies, then defeat him in battle.
Complete Stage 27 of the Subspace Emissary and R.O.B. will join your party.
I would either play 160 matches in Brawl Mode or go to Stage 27 (the plot twist) in the Subspace Emissary. 250 Trophies would take a bit long, and you could get many duplicates.
Play 250 matches in Brawl mode, then defeat him in battle.
Beat Target Smash with 30 characters on any one difficulty level, then defeat him in battle.
Mr. Game & Watch will join you when you defeat the boss Duon in part 25 of the Subspace Emissary.
Game & Watch is probably one of the last characters you unlock via Target Smash. I would beat Duon, go to Stage 25, and not have to do a battle. Win-win?
Play 300 matches in Brawl mode, then defeat him in battle.
Beat Classic Mode on any difficulty with 10 characters, then defeat him in battle.
Play over 10 hours in Brawl mode.
Complete Stage 31 of the Subspace Emissary, and Sonic will join your party.
I would either go to Stage 31 of the Subspace Emissary to activate a really awesome cutsecne, play 10+ hours in brawl mode (just leave 2 human controllers out all night), or beat classic mode with 10 characters. Good character comes easy.
Play 350 matches in Brawl mode, then beat it in a match.
Complete the Subspace Emissary, then return to Stage 18. At the end of the level where you must shoot yourself through barrels, stop on the platform below the new door. Go in and beat Jigglypuff.
Complete the Subspace Emissary, then complete twenty single-player Events on any difficulty and beat it in a fight.
Do the 20 events. They are easy to do, and you can always just do them on easy.
Play 400 matches in Brawl mode, then defeat him in battle.
After completing Subspace Emissary, finish Classic mode with any character on any difficulty, then defeat him in battle.
Complete the Subspace Emissary, then return to Stage 9. At the beginning of the level after the first mini-battle with Primids, a new door appears. Go in and beat Toon Link, then finish the stage.
I would complete the Subspace Emissary, beat classic mode with any character, then go and unlock Toon Link. This is very easy to do (not the Subspace Part, but classic mode) and you will get Toon Link easy.
Play 450 matches in Brawl mode, then defeat him in battle.
Beat Boss Battles mode with Fox or Falco, then defeat him in battle.
Complete the Subspace Emissary, then return to Stage 14. At the end of the second part of the level, go to the bottom of the elevator shaft instead of going into the door and into the new door that appears there, near the giant Bryans. Beat Wolf, then finish the stage.
The best way to get Wolf is to unlock him in the Subspace Emissary.
She comes from Daigasso! Band Brothers. When she plays her guitar, the soundwaves hit your opponents. To unlock this trophy, unlock 25 songs.
Gray Fox
Frank Jaeger, the Gray Fox, comes from the Metal Gear series to slice your opponents in pieces. He can also reflect projectiles. To unlock this trophy, unlock Snake.
Isaac
Isaac from the Golden Sun series appears and uses the attack force which appears as a giant hand that tries to push the enemies off the stage. To unlock this trophy, play 200 matches in Brawl mode.
Ray Mk III
This custom robo from Custom Robo flies around the stage, shooting energy blasts and missiles at opponents. To unlock this trophy, play 100 matches in Brawl mode.
Shadow the Hedgehog
Shadow from Shadow the Hedgehog uses the Chaos Control to slow the speed of every enemy. To unlock this trophy, unlock Sonic.
Tanks & Infantry
Many little soldiers and tanks from the Advance Wars series will march on the stage firing against your opponents and chasing them. Tanks will do more damage than infantry. To unlock this trophy, play 300 matches in Brawl mode.