I don't know Nevri, but personally? I view Mifuyu as Yachiyo's ex, they were together for a long time and went through a lot together, but as they grew older and into maturity, they became too different to stay together, Yachiyo is fine with being a meguca for the rest of her life, even with the risks and complications, she knows it is the life she choose and it is the fate she has accepted, Mifuyu has for a long time now yearned for a way out, a way that she could just lead a normal life without having to risk herself fighting Witches just to stay alive.
Mifuyu as Yachiyo's ex also fits too well with that one sequence in early chapter 6(later half of episode 11 to anime-onlys, keeping in mind that in the game, Mifuyu went there unprompted) where Mifuyu goes to the villa primarily to taunt Iroha and to mess with Yachiyo, that was completely Mifuyu acting as a bitchy ex, and going out of her way to rub on Iroha's face that what she has with Yachiyo at that moment doesn't compare to what she had with Yachiyo in the past... no wonder that Iroha is driven to make that major declaration to Yachiyo later in the chapter, Mifuyu made her want to go a step further.
Also can I just say, that one scene drove me very close to hating Mifuyu, she redeemed herself more than enough in the following chapters, and the story makes it clear that that scene, bitchy and venomous as it was, was ultimately just an act and that she really isn't like that, but it shows Iroha has the patience of a saint that she didn't force Mifuyu out of the house at crossbowpoint, I know I would've.