O-
but out of all seriousness, Rio did it, not the little Niha/Koyuki
plus Aris is the Princess of Nameless Priests/Gods and is supposed to take over Kivotos (their world) Kei is the Evil part of her/the one who's supposed to guide her actual path instead of being a student in GDD (but I think she turned into good in later volumes). In GDD (Game Developer Department)'s language, we can call her "Demon lord".
but you know.. via the power of friendship, the GDD and sensei/Player rescued Aris.
"It's ok to choose who you want to be"
and yeah, they were able to knock some sense into Rio as well, making her drop the president of seminar role.