>Eris is potentially chesty and beautiful and comes from nobility, but is really difficult to approach and has trouble actually being a decent person to others (without Rudy or someone else already close to her around).
Read the Eris Gaiden. Her entire story that takes place off screen is her basically becoming even worse and then having those aspects tamed by her own struggle to become a stronger swordswoman.

Also, big hint here, the most noted Greyrat family trait is their...affinity for kemonomimi girls.