I come from Smash Bros and when I hear the term main, it was used to generally say "This was my main go to guy for any situation; I could rely on him/her and only him/her" Now when I hear "Mains", it doesnt make a whole lot of sense as main is really one. You have a secondary but not multiple as Justin Wong said a while back...
Having a secondary character is a very good thing, because there will always be a bad match up for your main fighter. The only problem is that most of the time, your secondary character might not be on par with your main character.
The main point of me throwing this out there because I truly believe the word is being misused or misunderstood. Have a secondary if you you want but please not muliple characters make that character a projection of yourself
Character that you're best with In the majority of situations. Character you play with the most. That's what I think "main" is, so I classify myself as a parappa main. Kinda like your go-to-character
I interperet main as the character a person uses the most, and are most skilled with.
Secondary I consider the character that a person uses most frequently when they don't use their main. But some people may use their main so much, that they don't really have a dedicated secondary, so they may say they have more than one, if they switch between a couple characters when not playing their main.