>>542642406
This story takes place way before volume F
This story takes place before even volume 2

Rio and Himari have always had a "relationship"
Partially because Rio is autistic as shit and doesn't understand unless you're direct, and she constantly does stupid shit while barreling ahead without listening to anyone
Rio is the only one Himari is rude towards because she kinda deserves it, they have a deep history together. Once upon a time Himari, Rio, and Chihiro were very close, but Rio slowly became more authoritarian and started studying ancient gods, she became president of Millenium and immediately started spying on everyone in Millenium. Which earned her the nickname big sister based off 1984

Chihiro and Himari started veritas as a protest against Rio.