She was ok, until she became a vampire. Then she became a rotten no good fuckhead.
It was ok for her to be friends with Shido when she was human and he was vampire, but it wasn't ok for Shinichi to be friends with her after she became a vampire. Obviously she chose not to be friends with anyone from her school from choice.
Riho was also very jelous of Yayoi. She didn't like her makinf friends with Shido. It doesn't matter, Yayoi wasn't an ugly piece of shit like Riho was.
Riho was a worthless person and should had never come in the world!