Beatrice is a petite girl with long cream colored hair that she has tied in two twin drills along with blue eyes that have a butterfly pattern at the center. She wears a frilled dress with pink ribbons, tights, and shoes. A tiny crown lays latched upon the rear right side of her hair, with a butterfly-shaped ribbon joining her cape at her sternum.
Beatrice generally looks down on others and is bad at dealing with people who try to get close to her. Despite being full of bravado, she's a lonely person, and is softhearted beyond her perceived stubbornness. She is affectionate towards Puck because of their similarities and favors him over the others in the Roswaal household.
She has a distinct way of speaking, adding "I suppose" (かしら, kashira) and "in fact" (のよ, no yo) at the end of some of her sentences.
Around 400 years ago, Beatrice was created by Echidna. She helped her with her experiments, such as the Sanctuary Barrier, though she wasn't told everything as she didn't know that Lewes Meyer would be sealed in a crystal. Sometime later, Echidna ordered her to stand guard over the library while she waited for "that person," an order which she diligently followed.
Magic User: Beatrice is a master at Yin Magic and is capable of casting all Yin Magic, even those that have been long since forgotten by people in the present. Her power is far stronger than Subaru's own Yin Magic and she is capable of doing various things through it such as teleportation, freezing time, tearing space, and fixating her existence. However, since she is currently low on mana, she can't use most of her magic.
- Shamak (シャマク): The target becomes unable to sense where they are. However, they can still feel the ground and sense what's happening to their body.
- El Shamak (エル・シャマク): Beatrice's target becomes unable to control their bodies while under its effects.
- UI Shamak (ウル・シャマク): UI Shamak covers the area in darkness like Shamak, except the opponent can't tell anything while under its effect, such as where they are sitting or standing. Those affected by it can be snapped out of its effects.
- Al Shamak (アル・シャマク): Al Shamak is capable of sending its target to a different dimension.
- Door Crossing (扉渡り Tobirawatari): Beatrice can connect the door of the library to any door she wishes to. She can also use a limited version to teleport things.
- Infinite Corridor: Causes a spatial loop where the target or targets will wander through a repeating hallway until they find the right door.
- Minya (ミーニャ): Beatrice's magic that creates stakes made of crystallized mana covered with light purple flames. They are strong enough to penetrate defenses. Any stakes in the air explode and release smaller stakes that fly in all directions which then rips its targets apart.
- El Minya (エル・ミーニャ): Same as Minya, except the number of stakes increases to 40.
- UI Minya (ウル・ミーニャ): Beatrice creates a purple ring of light around the target that shrinks and binds them, subsequently spreading across their body. Once the target is bound, a huge purple-blue light appears above them to destroy them. The effect is currently unknown as Beatrice's attempt was stopped partway through.
- Murak (ムラク): Beatrice reduces the effects of gravity. Just by jumping lightly, a person can jump as if they had springs. If Beatrice wishes to, it can also be used to fly, although Beatrice will get blown around by the wind like a leaf, but they won't flip around in midair.
- Healing Magic: Beatrice summons water mana to heal the wounds of herself and others, however it only affects physical ones. Power varies by the skill and talent of the individual or partially that of the spirit if the caster is using Spirit Arts. Beatrice is highly skilled in this to the point she can regenerate lost organs.
Decurseifcation: She can detect and remove curses.
Mana Drain: Beatrice can absorb mana from any target via touch and instantaneously transfer it to herself.
Contract Spirit: Beatrice is currently contracted with Subaru, though according to her, he isn't a true Spirit User. She also has a terrible mana consumption rate and formerly got around this by draining mana from people in the mansion while using the library as a medium. As she is an Artificial Spirit, she also has several conditions Subaru is forced to follow:
- Subaru cannot form a contract with other Spirits.
- Beatrice cannot restore mana on her own.
- Beatrice cannot use the mana in the atmosphere.
- Beatrice can only drain mana from Subaru.
- Her appearance is set. So even if she changes her hairstyle, it will revert back to the drill-like pigtails.
- Beatrice shares the same attribute with Subaru, which is why they're highly compatible in terms of being a spirit and contractor.
- She used to rarely show up for dinner, but ever since Subaru arrived and after events of the demon beast incident, she has been showing up more frequently.
- As a spirit, she doesn't need to eat, but she does enjoy the pleasure of it.
- The author has stated that her birthday is a spoiler.