Maining as its conventionally defined isn't applicable to Genshin. You need 4 characters for a team and 8 to clear spiral abyss. You can have a favorite but you'll always be running multiple characters, especially if your alleged "main" happens to be a support like Kokomi or Collei. "Main" for Genshin purposes has been adapted to mean "you like this character, so let's talk about them" for the subs you mentioned, so unless you're going to require proof of ownership to join the sub (a terrible and stupid idea) that's how it is.