Unexpected

Once you pass Lv400, they become less useful, but I think it'll be tough if you don't have Florence at this stage. In events where characters are rewards, Cordy and Florence can be exchanged alternately, so I think it's better to prioritize exchanging for them over characters with black borders. Since the characters you're currently using are still viable, they're worth investing in.

Wow, you're so lucky to get so many Natas from the gacha lol. I only use Nata as a temporary substitute when I don't have Cor or Flo early on. So, anyone still using Nata in the late game is just a hardcore stan, I guess (´-ω-｀;) As for Flo, who was mentioned, she's better than Cor in early game progression, guild raids, and when enemies have artifacts or similar in quests. Flo is definitely superior to Cor in the early game, but since it's impossible to switch attackers frequently for free-to-play players, I recommend Cor, who you can take to the very end.