Do you have better suggestions, then?
Let's see, Rock could work for Dixie, because of her guitar.
Sora there's a lot you can do, I'd go with something Disney-related like Mouse or Princess that would make sense yet still be only vaguely related.
I'd use Think as a reference to Phoenix, you know, the Thinker statue.
I like the idea of using Attack for Lip but I don't know if the Japanese devs would be familiar with the Tetris Attack name, so maybe Magic? Generic, for sure, but it still fits. Or, since I'm on a Disney role, Tinker, as a reference to her being a fairy (hey, it's no more of a stretch than Doyle for Arsene).
Isaac is hard because you know people are going to be looking for anything related to him. Maybe Wise, to reference the Wise One. Or just Planet, as the Adepts are named in reference to alchemical symbols (which are named after planets, which are named after Roman gods, I guess).
Octoling I struggle to think any Splatoon related term that wouldn't be a dead giveaway. Maybe Bond, because of Agent 8? Something like August (the 8th month) or Oxygen (8th in the periodic table) is vague enough, but if the connection is made to the number 8 it's a dead giveaway.