You should only use one character in fighting games. That way you're experienced with every matchup without having to switch. Even if the character you like isn't "good", you'll still get very good at the game quicker than players who are always changing characters. It is good to have one other character though for variety, and so you don't have to worry about constant mirror matches.
Having one main and one backup is the best strategy for fighting games especially Tekken since there's so many moves and things to learn. It's easier to learn more characters in MK, but muscle memory is still important.
When I play I don't have to think about anything other than out playing the other guy. Every combo and movement comes natural. I always destroy the players that have several hundred matches with 5 or 6 different characters, but the players that use one are difficult to beat.
Off topic: but this is why MMA fighters have no foundation, and get their asses handed to them in boxing and sometimes K1. They try to learn so much shit, half of which is unimportant, that they can't setup and throw a proper punch.
TLDR: Call a plumber to fix your sink, not a carpenter that knows a thing or two about plumbing. Sticking to one or two characters that you like is funner and better for muscle memory. Less is more.
