A powerful witch and yet another girl that loves Tsukune.
The first out of the group to meet Ruby, is Yukari. As Ruby had been deliberatly luring the younger witch into the field. Ruby starts acting nice... but then the conversation heads down a more evil road. Ruby wanted Yukari to join her in the field, living in peace while killing any and all humans that happened to enter. Yukari of course delines and instead, invites Ruby back to the tent that everyone is staying in at the time. All of them are extremely kind, but Ruby still doesn't feel like she belongs, so she flies back to the mansion in the center of the flower field that the "master" lives in. The odd part is this, Ruby's "master" died of old age long before Tsukune or the others ever showed up. The "thing" that Ruby almost mindlessly serves is merely a construct of her imagination.
After a while, Ruby shows up at Yokai academy taking on differant jobs for each, almost twice, episode. In response to the ever presant questions about why she has switched jobs she always replies (annoyingly) with "many, many things have happened." She says that line without fail every episode that she appears in after the sunflower field.
Part of the reason for her numerous job switches, Ruby is obsessed with cos-play. Somthing that usually rouses up a laugh or two.
Ruby has one episode dedicated to her, one where she tries to enforce a school rule that the skirts on the girls must fall ten centimeters below the knee. The irony of this is that a passing breeze renders her entire plan of "lessening the erotic nature of the school" entirely useless.
Now before I say anymore I must explain somthing about Ruby's standard outfit. It is a long, old-fashioned dress with an apron.
When Ruby's plan fails she attempts to rush foreward and cover Tsukune's eyes... she trips on an unusually positioned rock, falls flat on her face , and her skirt flips up and forward. Revealing somthing white, frilly, and skimpy. That moment right there is the primary ecchi moment involving Ruby, although she is not above the occasinal breast rub that Kurumu usually initiates and the others (with exception of Moka) then join.
Ruby may also have feelings for Ginei, although this is really never explored outside of a few small moments.