Beatrix is tall, slender, and curvy and has chestnut brown hair. She wears a silver eye patch over her right eye, with the left being a purple/reddish color. In the original concept art, she has a bandana over her right eye, and the left eye is blue. She frequently tosses back her hair with her hand. She wears less armor than the heavily clad Adelbert Steiner, and thus is more agile.
Beatrix at first views Adelbert Steiner, the Captain of the Knights of Pluto and a warrior in her senior, as a rival. Beatrix is bitter and Steiner is infuriated over the superiority of Alexandrian soldiers led by Beatrix over his Pluto Knights.
Beatrix is noble, proud, and most of all, loyal to Alexandria and the Alexandrian royal family. She is confident in her ability as a soldier, and can be seen as being arrogant because of this. Beatrix has an outstanding ability as a leader, collecting loyalty and admiration from both the public and the soldiers under her. Beatrix is soft-spoken and addresses people with perfect manners, be it to her soldiers or to an enemy, although this is not as obvious in the English translation, as it is in the original Japanese version.
Beatrix is first introduced as the game's tertiary antagonist, helping Queen Brahne conquer the other kingdoms of Gaia. She is a fierce and skilled fighter, claiming to have once single-handedly killed a hundred knights. She has a rivalry with Adelbert Steiner, the captain of the Knights of Pluto (Alexandria's only male soldiers); at one point, they fought each other and he defeated her by sheer luck. Beatrix thinks little of Steiner, who in turn envies her position and angrily laments her troops' superiority over his.
At the beginning of the game, Princess Garnet runs away by deliberately letting the Tantalus Theater Troupe kidnap her. The twin jesters Zorn and Thorn report her disappearance to Beatrix. While Zidane Tribal and his allies oppose the Alexandrians, they fight against Beatrix three times in total, but easily defeats them every time. All the while, she expresses displeasure at Brahne's use of eidolons and black mages - the latter supplied to her by her mysterious arms dealer, Kuja - instead of the Alexandrian army.
After Garnet has been recaptured, Zidane and his friends rescue her, but are caught by Beatrix, who soon has them at her mercy. However, Zidane then shows her Garnet, who under a sleep spell cast on her by Kuja. Beatrix had earlier learned that Brahne was going have Garnet executed, and while she did not believe it at first, seeing Garnet's condition convinces her. Bound by her duty to protect the princess, Beatrix draws the line and turns on Brahne, and along with Steiner, the Tantalus Theater Troupe, and Zidane's friend Freya, covers for Zidane and Garnet while they escape. Shortly thereafter, Brahne is killed by Kuja at the Iifa Tree.
With Brahne dead, Garnet succeeds her as queen with Beatrix and Steiner to be her protectors. While Garnet prepares to take the throne, Zidane's friend Eiko tries to send him a love letter, but Beatrix ends up getting instead and thinks it's from Steiner, who later finds it himself. As a result, the two begin a relationship. On the eve of Garnet's coronation, Kuja returns and attacks Alexandria with the eidelon Bahamut, and Beatrix fights alongside Zidane's party and Steiner to defend the city. Kuja's master, Garland, intervenes and devastates Alexandria to keep him from getting any more eidelons, with which Kuja was planning to overthrow him. Beatrix stays behind while Zidane's party goes after Kuja.
Later, after Kuja has killed Garland and returned to Gaia, Beatrix and Regend Cid of Lindblum arrive to cover for Zidane's party as they enter Memoria in pursuit of Kuja. After Kuja's defeat, Beatrix is notified of the heroes' return by Mikoto, a girl of Zidane's race, the Genomes. The assembled characters reunite and depart, except for Zidane who disappears after trying to save a dying Kuja. Sometime later, Alexandria has been rebuilt and Garnet has been crowned queen. Beatrix decides to quit her job and leave, but Steiner stops her and convinces her to stay. At the end of the game, while Tantalus performs for Garnet, Zidane makes a surprise return and, after allowing Garnet to run out and see him, Steiner and Beatrix raise the latter's sword, "Save the Queen", together as Zidane and Garnet embrace.