Good Heavens!. The way she talks is quite amusing. I am going to have her speak in a posh British accent in my head from now on. Should this ever get an anime adaptation with an English dub, if she doesnt sound like a posh British girl I will be very sad.
I offer for your consideration
"British Ponies"
I think what is needed though is an American FAKING a British accent, just slightly off - and occasionally forgetting when angry.
By contrast though, we will have a genuine Ojou-Sama and even the Emperial Family later on, who should be dubbed in more reliably upper class British. It's a shame Penelope Keith is no longer with us....
She's a dumbass but she's also the most honest lovable person for Mafuyu.
Mafuyu is, if we look at it clearly, a victim of child abuse. She is an orphan, thrown into a dungeon at an early age (despite, we are told, the Japanese government making it illegal for minors to enter the dungeon under 14) and intensively trained/indoctrinated to be part of a government "black ops" programme. Now, forced to retire by injury that could take years to heal (if at all) she is trying to build a new life as a normal schoolgirl - despite all the mental damage that role must have caused, and the loss of a role she had literally built her life and personality around.
Then we have Karlin, who for her faults, has created an achievable goal for herself, and genuinely wants to see her classmate thrive. Given we know what the situation is NOW, clearly Mafuyu is gonna get dragged into Karlin's orbit just as she needs to remake herself, and a big part of how she defines herself going forward will be in relation to how she guides and channels Karlin as a "big sister" type.