Writing Ether, I've constantly struggled on how I could give each character a unique voice. Have you all noticed any of these?

Setsuna: Trails off a lot (...)

Yelena: Says "like" often

Margot: Never uses contractions, never uses profanity, speaks French when upset

Oliver: Vulgar expressions

Ancil: Speaking french/"dignified" language

Eustis: I feel like he doesn't have any strong speech quirk. (Should I change  that?)

Megumi: She should come across as sarcastic in a playful way, with some lighthearted puns/jabs at Setsuna

Janus: Boring and blunt

Laurel: Apathetic and reserved

Dialogue is one of the key parts of defining a character's personality because it illustrates how they interact with others. Hopefully, you notice things like how Setsuna talks differently to Megumi than she does to Yelena, or that Margot doesn't curse and is offended by dirty language. This seems simple, but is surprisingly difficult. The larger the cast, the more difficult it is to get unique voices. Some characters, like Janus and Laurel, are more defined by there lack of dialogue; that's why other details are important as well - but that's another post!