the first thing to note is that mortis is skilled at imitating other people, meaning that she can sound incredibly verbose/well spoken, especially when she imitates sakiko. she uses literary, flowing terms mutsumi never did

also like to note a detail here: when she says she'll talk to the producer for nyamu, it's a pretty much 1:1 replication of what minami said last episode lol she has the imitating other people thing down to a T

when mortis just speaks as herself, though, she sounds much different than mutsumi. mutsumi generally speaks in fragments of sentences, and thus what she says doesn't have much personality. she basically just says the bare minimum to get her thoughts across. meanwhile, for mortis

she sounds much more like your average young-teens girl. she speaks in full sentences, using more end particles like の in a feminine way. she also uses words/structures like だって, -ちゃう, and もん that offer a more youthful air, and she drops particles very often

additionally, when speaking to mutsumi in her doll form(not fronting yet), mortis uses very gentle, coaxing language. she uses a lot of よね and ね's to soften her tone.

as herself, mortis also defaults to more childish language as well. for example, she uses the words お話 and お人形, adding the honorific お in front of the words 話 and 人形, which is sounds a bit childish. there's also the scene where ave mujica breaks up,

in which mortis has an outburst where she says ヤダヤダ！絶対ヤダ！解散はイヤ！ which, along with the body language, is a very childish way to express your dislike. also, in episode 6, when soyo goes to leave, mortis shouts ダメ、行っちゃイヤだ！which again, sounds very childish

in general she uses the words だめ and いや often, which aren't necessarily childish themselves, but shows that mortis generally doesn't use very complicated or advanced vocabulary(when being herself) and defaults to these simple, vague words.

this sets her apart from say, sakiko and umiri. if anything i'd say she's like uika, almost, where what she says is generally very "raw" and she does little to cushion her words. in terms of verbosity/properness i'd say it's mutsumi -> mortis -> uika -> nyamu -> sakiko/umiri

Woah this kinda blowed up stream loftmoon’s ave Mujica fansubs and look up how to do it if u don’t know how https://nyaa.si/view/1927858