Rossweisse and Issei initially had a student-teacher relationship, where Rossweisse is shown to be giving advice to Issei on various topics, from Issei's studies in school, training and even his relationships with the girls. She eventually gained some slight interest in Issei during Volume 12, where she and the Occult Research Club members asked Issei out on a date.
In Volume 17, she asks Issei to be her boyfriend due to her grandmother coming for a visit, to whom she lied about having a boyfriend. Later they go on a date to Tokyo which is interrupted by the sudden appearance of Euclid. Worried that she might be used by Euclid to hurt her friends, she asks Issei to kill her if that were to happen but Issei angrily disagrees, saying that he will protect her from him. During the attack on Auros and Agreas, Issei does rescue Rossweisse from Euclid, defeating him in the process. By the end of the volume, Rossweisse develops feelings for Issei, even asking him out on a date to a 200 yen shop next time. In Volume 22, under Rias' orders, Rossweisse was transferred to Issei's peerage, becoming his first official Rook.
Rossweisse is Rias' second Rook and is shown to be very loyal to her. Rias trusts her capabilities, so much so that she thought it best to transfer her to Issei's peerage.
Originally, she was working under Odin as his bodyguard and had to stop him many times for his lewd behavior towards the women that he met but later on was discarded by him and at this time, she does not get along with him very well.
Her grandmother who raised her while her parents rarely come home. Göndul tends to be overly worried about Rossweisse, claiming her to be clumsy as well as scolding her for making rash decisions on her own such as leaving home, becoming a Devil and becoming a teacher. Göndul always tells her granddaughter to get a boyfriend so she wouldn't have to worry about her, which Rossweisse lied about saying that Issei is her boyfriend. Göndul often refers to Rossweisse as "Rose".