You could complete classic mode with the 14 characters you started with, or Play Multiplayer Vs. mode a couple of times until he shows up "don't count on it"
He will then challenge you to a fight, if you win he's yours, if you lose he'll challenge you another time.
Marth has to be the second easiest character to get (for Ness you must complete 5 brawl matches). To unlock Marth, complete 10 Brawl matches, battle him and you got him.
Beat classic mode with 13 of the original characters marth should appear at the end of classic mode defeat him to unlock him.
Unlock marth then use him to beat classic mode.
Play as Marth on classic mode (no continues) or play 900 vs matches.
In SSBM, you have to beat him with Marth after you finish classic with Marth.
Get marth and beat classic mode with him
side B
You can't, you can only unlock him in SSBB.
There is 3 ways to unlock him: 1] Complete Classic Mode with all 14 starter characters. 2] Play at least one vs match with all the starter characters 3] Play 400 vs matches. Marth will then challenge you on the stage Hyrule temple. Have fun :)
First unlock marth to do that beat classic mode on any mode with 13 of the original characters defeat marth to unlock him then use marth to beat classic mode roy will appear defeat him to unlock him.
Finish the subspace emissary.
You can't unlock Shadow in SSBM. You can unlock Shadow in SSBB though, he is an assist trophy. In SSBB, you can use a texture hack on Sonic to make him look like Shadow and you can play as him.
Use Marth on Classic or Adventre then beat Roy just a reminder you have to unlock Marth too
too bad it doesnt exist