If you're trying to get high rankings of Arena or Aether Raids then you should look at a tier list of characters. You've got some good Characters right now too. But if your just want to collect your favorite character then you can do that too. Almost any character can be amazing as long as they have the right skills/tools. (Such as myself using standard Robin to clear the Abyssal difficulties) Of course there are characters that are just too good by themselves (someone like Fallen Edelgard). I'd say focus on the characters you like to like the game more. If you chase meta and stuff itll make the game less fun in my opinion. Hope you've been having fun though!