Beatrix von Lothringen is the fiancee of Kai von Granzreich.
Beatrix has a rather boyish look, due to tying her long hair back and wearing men's clothing instead of dresses. She is tall, and has straight blond hair. She is quite tall for a woman.
Despite being a woman, Beatrix is considered to be a true gentleman due to her boyish looks and gallant, friendly behavior. She acts like a big sister towards the princes due to taking care of them when they are little. She doesn't like to wear dresses and instead wears men's clothing. Underneath her tomboy exterior, she's actually prone to worrying about Kai not seeing her as a woman and his fiancee.
Beatrix is from an important noble family and was engaged to her cousin Kai by their parents when they were both fifteen. When they were both young, they often took care of the younger princes when the eldest prince was away.
Beatrix came to the palace one day for a picnic at a national park, and she, like many others, initially thought Heine was a young boy. Heine also thought she was a young man, and was surprised when Bruno tells him that she's actually Beatrix von Lothringen, a cousin from their mother's side who's also Kai's fiancee. Beatrix didn't seem offended by Heine's mistake since she has often been mistook for a boy due to her preference of men's clothing. Licht mentions that she and Kai don't seem like a couple at all, something that Beatrix is keenly aware of, which is why she had suggested the picnic to Kai in the first place, even making a lunch for him. Unfortunately for her, Kai assumed that they were inviting everyone else. However, she is still hopeful that they could have some time alone together.
At the park, the princes were confused as to how they are having the picnic, having never eaten on the ground before. Beatrix had to show them how to spread the sheet, and while doing son, Heine remarked to her that she seemed like their elder sister. She explains that she looked after the princes when they were younger along with Kai when the eldest prince was away, and recounted some embarrassing stories about them. After the picnic sheet was laid down, Beatrix prepares to give Kai her lunch, but he brought out the lunch that their grandmother already made for them. The princes are impressed that she made a lunch herself, but Leonhard hates it because it has his least favorite peppers and carrots in it. He then proceeds to sneak them into Kai's plate when he's not looking, but ends up getting caught by Heine and Bruno. Beatrix tries to show her feminine side to Kai again by wiping his mouth with a handkerchief, but he gets distracted by small and cute rabbits, reminding Beatrix that she is not small or cute at all, in her opinion. Heine being next to her only reinforces that thought, and she can't help but notice how close the two look. Kai makes a flower crown that ends up being for Heine, much to Beatrix's despair. To make things even worse, it starts to rain, so they're forced to cut the picnic short. While trying to dry her hair, Beatrix regrets setting up the picnic in the first place.
Back at the palace, the Queen Mother offers everyone a change of clothes. Beatrix ends up taking the longest to change, because she was given a frilly dress that she thought she'd look absurd in. The princes seem to like her new look, but Beatrix is still extremely embarrassed. When she goes to the carriage that's going to take her home, she bumps into Kai, who didn't say anything about the dress, making her despair again about whether he actually cares or not. However, he interrupts her thoughts by giving her a flower crown he made and telling her that he had fun today. He also tells her she looks pretty. Delighted, she tells everyone to do this again sometime. The carriage heads off as she thinks hopefully that one day she and Kai will start acting like a real couple.