Erika is extremely annoying. And Hiro seems quite devious, she obviously wants to monopolize nagi with the whole setting up annoying friend with Erika.
Damnnnn. I figured Hiro out. She has obsession with being first. Including being first in Nagi's heart. Hiro doesn't like Nagi, she likes winning Nagi's affection. She likes being first out of her, imouto, and Erika. Dude.