
Alongside the research about evolution (in somewhat sci-fi way), they stumbled upon a problem that made that research to be halted.
To let the research to be able to continue to a degree while presering some other specimens, some researchers left, conducted some experiment in the hidden facility.
At some point Yuzu-senpai's big sis and Akky original personality, Madarame made some compromise.
Big sis will act as 'mother' who sustains all other specimens on the island, while the side effect of those function was the creation of ghoul-like clone.
While, Madarame will be 'expelled' from the island to return later after the conditions stabilze.
Currently it's not clear whether the experiment was about cloning, personality injecting, or something else.
Well, I mean Madarame was already an adult at the time but as Akky, his memories started during a toddler age-wise.
So, did he consume a medicine like Conan or he cloned himself then transfer his memories to the clone?

From the prologue I thought the name sound Chinese, but
on Ch.1 and afterwards it became Korean??

I think it's originally in Korean (raws here: https://toonkor.com/우주의-별).
How could Lucy use Aquarius-nee-san magic while she hadn't contracted her again??
Oh Juvia, you silly girl...=w=
That was the grace of Celestial Spirit King... if I remembered it correctly.
He 'bestowed' Lucy with that power to protect herself -So that she won't be drown in sorrow of losing her contract with nee-san; tho I don't know about the condition/requirement of usage after the battle ended.