I forgot about toon Link his only difference is that he has shorter range. Mario and Luigi are vastly different though. In Smash Bros they only have one similar attack which is the fireball and even with that Luigi's fireball goes straight even when shot in mid air. Mario's bounces and falls while in the air. Their Ultras are completely different. Luigi's is just out there.
Ken and Ryuu on the other hand not only look similar in appearance but also play the same. back in SF2 they were completely identical in SFIV all I see different are their supers and ultras.