The Mukami family's past. I do not plan to be overly in-depth about their time in Romania since they briefly resided there. Nor will I delve much into any cultural aspects, especially since it seems like it's not entirely relevant to the games. I'm trying my best with the research I am doing.
It's revealed that Yuuma's original name was Edgar and Kou's was Emilio. However, Azusa and Ruki never mention other names. I may be wrong, but I feel like they'd likely have original names too, so I have decided to do some research and choose my own. Ruki's original name was Severin ( seems to mean stern or serious ), and Azusa's would be Alexi ( seems to be a form of Alex, but I like that it seems common, given Azusa's background of being abandoned. I doubt much thought went into his name ). So:
Ruki: Severin
Kou: Emilio
Azusa: Alexi
Yuuma: Edgar
There is no guarantee Ruki and Kou will reveal their former names, but if they choose to, it will only be with people they have built deep connections with and trust. Ruki will sternly ask you not to use his former name, as he prefers not to be associated with it anymore. Kou will likely lash out if you call him his due to his past trauma. Telling you doesn't grant automatic permission to use these names. Yuuma can't remember, so unless Shuu is involved, it likely won't come up. Even so, Yuuma has no interest in being Edgar anymore ( he merely wants to recover his memory ). And Azusa really doesn't care, but prefers Azusa. Compared to his brothers, he has the least negative association with it. I will be headcanoning that Karlheinz gave them these names after saving them.
After turning them into impure vampires, they likely stayed in Romania for at least another year, until they were able to control themselves as new vampires. When they went to Japan, Karlheniz told them they would have a greater purpose in due time, but first, he had them educated. They were home-schooled for a period of time. Possible chance they didn't attend with other students until they were in junior high.
As a child, Ruki was the most privileged and educated since he lived with his wealthy family for a time. Yuuma was likely educated too, but to a lesser degree. With Azusa being abandoned, I highly doubt that receiving an education was a high priority or easy. He may have learned things on the streets. Despite coming from an aristocratic family, Kou ( as far as I know ) is unaware of this. His parents gave him up since they couldn't care for him, and the maid who took him in was also unable to truly care for him. I really think he was likely undereducated, like Azusa. And due to Kou's mistreatment at the orphanage, even if they had a teaching program, he probably wasn't able to attend or retain much.
When the opportunity came, all four boys were finally able to learn to read and write. Ruki is the smartest of the Mukami's. He tutors his brothers if they need extra help and is a big reason why they never forgot how to speak Romanian. This may end up in another post about Kou, too, but Ruki is the one who taught Kou how to take care of himself, like brushing his hair, his teeth, dressing properly, and tying his shoes, among other things. I do believe Kou was Ruki's main student.
And to end this off with a little extra. All four of them can read, speak, and write in Romanian, Japanese, and English. Ruki excels in these. Azusa follows suit, though he has occasional mistakes ( his best language probably being Japanese ). Yuuma struggles a bit more with languages in terms of writing (reading and speaking are mostly fine ). I think sentence structure for English trips him up ( occasional spelling errors with big words and punctuation mistakes ), and he tends to spell words how he says them. And he has issues with Kanji and somewhat with katakana. Kou struggles a little with writing, too. He likes writing in English but has occasional punctuation mistakes and gets frustrated over big words ( sounding them out like kids are taught ). He loathes Kanji. He can read it and write it better than Yuuma, but sometimes he confuses what is what. Both Yuuma and he avoid Kanji if they can. When Kou actually has to write something himself without help, he'll just try his best with Kanji. His fans who receive handwritten things with kanji mistakes find it endearing and cute, but it upsets him when they bring it up.