Yuzuyu is the voice actress for koakuma which translate to (small devil), as she in the original game did not have an official name which lead to the fans giving her the nickname and later the the creator (ZUN) adapted the name as the official name for her.
She was in charge of voicing the role of koakuma and Mystia in the OG Cookie☆, she also served as an artist.
The nickname "Fucking fat aneki " was born when Renna Rio (HZN) who was infamous for the root of all evil, released a small devil in another work where she was appeared to be too fat.
When playing the role of Mystia Lorelei, she also left the line "無理無理！産めない！" which translate to "I can't do it! I can't give birth!".