Hiya, welcome to our community!
Well, if I was giving anyone advice on this, I would say to pick one character to focus on the most. Now, whatever character that is, if it is mid tier or higher, you can get by without another secondary or backup character. But, if it is a low tier character, I would advise picking up someone to compliment him/her. But honestly, it all depends on how good you can make your character. If you can turn a low tier character in a monster, then go ahead and main that one exclusively.
In my opinion, the best thing you can do is pick your favourite character on the roster, and just work to get good with him/her. More fun that way!