Kat is a pretty chill girl, who's also Ethan's best friend. 

She's a sort of older sister figure to Ethan (Slate), although they aren't related, they're just best friends. She's pretty low-key, and laid back. Kat's also a pretty passionate artist, and finds painting to be extremely relaxing, although she doesn't really care about the finished product. She's willing to correct Ethan pretty often, keeping him in reality.  She's a very go with the flow type gal, and mostly just enjoys tagging along with Ethan on most things. 

When voicing Kat, imagine a slightly older sister type tone, but keep her very relaxed and chill.

Also note that I'll also be taking how Ethan and Kat submissions sound when played together in order to test vocal synergy when casting, so do keep your delivery fairly natural.