Originally Odin’s bodyguard. After the battle with Loki and Fenrir, Odin leaves her in Issei’s town. She then gets persuaded by Rias to become her servant. She remains in Kuou Academy as a teacher.
Rossweisse is known to be a very serious person. She has a tendency to buy things at sales when they’re cheap, allowing Issei to coin for her the nickname “The 100-Yen Shop Valkyrie.”
Rossweisse makes her first appearance in Volume 5 as Odin’s bodyguard, reminding him of his status and telling him not to look at Serafall Leviathan lecherously. Odin responds by mocking her for not having a boyfriend.
She later reappears in Volume 7 during Issei and Akeno’s date, scolding them for being in the motel during the day and telling them that high-school students should stay home and study. She later helps in the battle against Loki and Fenrir and sealed Loki with her spells. She was abandoned shortly after that by Odin much to her horror, claiming that she was fired. She was then persuaded by Rias into becoming her servant, seeing all the benefits she can get by becoming a Devil and having a stable job of becoming a teacher at Kuoh Academy.
In Volume 9, she accompanies the second-year students to Kyoto. When Issei attempted to peek at the girls, Rossweisse attempted to stop him only to become another victim of Issei’s Dress Break, much to her horror due to the fact that her jersey (which was bought under discount) was spoiled. When Issei sees Azazel drinking in the morning telling him that it is not good for health, Rossweisse strongly agrees and lectures Azazel on the fact but was cut short after having being teased by Azazel on not having a boyfriend. Angered by that, she snatches his alcohol and drinks all of it, making herself drunk. This action, however, would indirectly cause her loss when she fought against Heracles.
In Volume 10, she and Koneko fought against Misteeta Sabnock and Gandoma Balam at the Rating Game between their masters, winning the match. She, Xenovia, and Yuuto Kiba would then face Sairaorg Bael but lost to the latter.
In Volume 11, Rossweisse goes back to Asgard to train after her defeat in Kyoto and in her match against Sairaorg but reminded Issei of his mid-term test and prepared a question paper for his reference before leaving.
In Volume 12, she returns after learning various defense magic from her country, which allows her to defend against attacks that are 10 times more powerful than Heracles’ Balance Breaker. She also told Rias and her comrades about the rumors that she heard in her homeland regarding the Vampires, that a certain noble Vampire house captured a Longinus possessor which lead to battles between Vampires. After Issei successfully passes his Middle-class Promotion test, Rossweisse like all the female members of the Occult Research Club, decided to go out on a date with Issei.
During the events of Volume 14, Rossweisse participated in the meeting of Mephisto Pheles in regards to making a pact with a Magician due to the Gremory group’s recent popularity. Later, she participates in the meeting with the Vampires from the Carmilla Faction. She, along with with rest of the Gremory group (excluding Rias and Kiba), later team up with the Sitri group to rescue Ravel, Koneko, and Gasper after the three were taken captive by the Magician Faction.
In volume 17 Euclid Lucifuge offers her to join him, as he is interested in her research about 666. He later tries to Kidnap her but is stopped by Issei and Rias. It’s revealed that Euclid was interested in her because she resembled his sister, Grayfia.
Rossweisse is highly proficient in the use of Norse magic, specializing in long-range and bombarding attacks. She is capable of using ice, fire, water, lightning, wind, thunder, light, summoning, and explosive based magic.
Upon returning from her training in Valhalla in Volume 12, she starts to use defensive spells (using the trait of Rook) which are strong enough to easily shield against Balance Breakers.