I wrote this not too long ago, to give advice to people getting into competitive Brawl when it comes out, but it applies here as well.
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Picking your character
When Brawl comes out, you're going to want to play as your favorite character. In fact, you probably won't want to play as anybody else. Personally, I prayed for Meta-Knight to be in "the next Smash" years ago, long before it was announced. When he was confirmed, I swore I would be the best Meta-Knight player in the world.
Fortunately for me, the few tournament players who attended E for All--with its playable Brawl demo--confirmed that Meta-Knight was a very good character.
This, obviously, was fantastic news for me. Not everybody, however, is going to be so lucky.
Some people will be wholeheartedly devoted to a single character. A character who is, by competitive standards, bad.
I'm not here to tell you that you shouldn't play as your favorite character. My recommendation is only that you remain flexible in your character choice. Play as multiple characters. Pick a character who, statistically speaking, is very good. If that happens to be your favorite character, more power to you. Play as characters who aren't. You will learn much more about the game by trying new characters on for size, and it will only help you become better with your favorite.
The prime case study for this is renowned Mewtwo player Taj, from Arizona. He is widely known as the best Mewtwo player in the world; at large tournaments he tends to have long lines of people waiting to play friendly matches with him. This is because Mewtwo is currently rated as the very worst character in the game.
Anybody who has watched him play in tournament, however, knows that he relies almost exclusively on Marth. While slightly less famous on this score, he is also considered one of the best Marth players in addition to his Mewtwo fame. Taj strikes a balance between playing his "true main" and his "tournament main," and is not only successful in tournament play, but he has the acclaim of a skilled low-tier user.
The second example to consider, however, is Chu Dat. Chu uses the Ice Climbers, who were considered by most to be very bad; by finding out new strategies with them, he invented a successful playstyle and places very highly in almost every tournament he attends.
Anybody who pays attention to Chu's play also knows that he's skilled with a wide variety of characters, however. He has been seen playing Young Link, Pikachu, and even Pichu in serious tournament play. Even though he virtually invented the Ice Climbers, Chu hasn't limited his horizons.
So even if you think your character is secretly amazing, or if you know it isn't and still want to use it, remain flexible. Explore multiple characters. Try playing as the powerhouses, the underdogs, and the unexplored. You'll find that:
* You can actually win and won't suffer from low-tier dejection syndrome.
* Playing multiple characters helps increase your general knowledge of the game, keeps your skill from stagnating, and helps you have more fun all around.
* You will be less susceptible to incredibly harsh counterpicking.
* Occasionally you can bring in your “true” main as a secret weapon or use it in weird counter-picks for a flashy and crowd-pleasing win.
* You might actually find out that a bad character is actually better than everybody thinks. It's not likely, but it could happen--the only way to find out is by trying. Just don't invest all of your time and you'll be less disappointed if it doesn't pan out.
In the end, just be realistic. If you think a "bad" character actually has great potential, then pursue it. The important thing is to be honest about their chances, and not to pointlessly limit yourself.