Learn a main character, after you have fundamentals you can learn a secondary or whatever. However, you should at least screw around with other character just to see their mindset.
Realistically, top players can do this mostly because of game knowledge of system mechanics and great fundamentals. In KOF for example, if you have really good fundamentals and know the system, you can use anyone since the cast’s combos really all work the same and start from the same moves (St.C, Cr.A, Cr,B, etc). That’s why you can see guys like Reynald, Romance, Woo and Xiaohai pull out Athena or Daimon from their ass and win tournaments.
You don’t need to learn the entire cast, just know the basic game plan of each character and how to punish them and their gimmicks. If there is a character you consistently lose to, going into training mode and using the character you hate can be eye opening and fun.