I am russian and I can confirm that this is accurate. 9th of May is a very big deal in Russia and is celebrated in a bigger manner than any other holiday, except New Years.
I doubt this has anything to do with Chika though, would've made more sense to make a Slavic girl (which we dont have) and give her this birthday, OR give 9th of May to Yumi because she's the favorite girl for most Russians.
Just to throw this out there, (No actual spoilers for where you are) Ami treated Sensei the way he taught her to. She's already seen him at his worst before, knows he wants to be left alone when he's unwell:
(She was apparently taught to milk him, and help him shit in a bucket)
On the other hand, Sensei's too much of a narcissist to actually take care of someone. He just treated her the way he'd want to be treated, except badly. Like Ami pointed out, she'd never expect him to be able to take care of her anyway:
(She's been marinating in her own issues while taking care of Sensei for around 8 years, hence her snapping. (Ignoring the trauma that the resets have also likely had on her))
Basically, they have a fucked up/"special" relationship where Ami acts more like an adult than he does, and that's not good for either of them, and almost no one besides kind of Maya and Niki understand Sensei's and Ami's relationship (ignoring the optional sex). Hence why both Ami and Sensei noted that others "don't get it":
Wonder why Tsukasa calls her sister "nee-sama" when that means big brother while "nee-chan" means big sister. It is something to do with power? Choosing a more masculine nickname because men are stereotypically more powerful?
Also, 'nee' is short for 'onee' which means elder sister, whilst 'onii' or 'nii' means elder brother... Void is probably making this mistake due to western pronunciation norms, 'onee/nee' sounds like 'neh' as in 'eh?', not 'nii', as in your 'knee', that's how 'onii/nii' sounds.
That's precisely why. Yumi is the personification of the russian highschool girl that is foul-mouthed but so cute that your average slav friend will inevitably fall in love with her, somehow achieved totally by accident trying to make a yakuza princess.
Rin used to be my favorite girl many years ago (still top 5) because she was very feminine and calm. What an irony that the most callous girl ended up growing on me more than her.
