Beerain is a bee-type Invectid and serves as the only female member of the Big Four, the elite generals in the army of the ruler Mantid. As a high-ranking officer, she commands swarms of insectoid soldiers known as Buzzrays in large-scale battles. Her physical appearance is distinctive among her kind, featuring pink skin, a red face mask, short orange hair, antennae, and large wings that grant her the ability to fly great distances. She wears a white skirt and has puff-like adornments around her hands and legs. Her primary weapon of choice in combat is a whip.

In terms of personality, Beerain is characterized by her use of wit and guile rather than brute force to accomplish her missions. She has little regard for honor in a fight and has openly reminded her opponents that she never fights fairly. However, over the course of the story, she begins to see the value in such honorable conduct. Despite her position within Mantid's army, her loyalty is not absolute, as she grows increasingly concerned over her lord's ruthless ambition and callousness over the years. She maintains a complex relationship with her fellow commander Buguese, with hints of romantic tension between them. Even after a rift forms between them when he refuses to believe her warnings about Mantid, she ultimately comes to his aid and saves his life.

Beerain's primary motivation for serving Mantid's cause is her desire to restore the beauty of the Invectid lands, which have been deprived of sunlight. Her commitment to this goal ultimately overrides her loyalty to Mantid himself. When she overhears Mantid revealing his true intention to seize all of the Oracle Key's power for himself rather than sharing it, she becomes a fugitive, barely escaping with her life after being attacked by Mantid and his ally Aqune. During her escape, her wings are damaged, and she experiences helplessness and despair, crying for the first time in her life.

In her role within the story, Beerain serves as a recurring antagonist who frequently employs deceptive tactics. In one notable mission, she attacks a village and renders the humans unconscious, then poses as a human girl named Rain to lure the Spider Riders into a trap involving hallucinogenic Blossoms of Darkness. Another time, she disguises herself as Prince Lumen's childhood friend Lady Noia in an attempt to steal an Oracle Key. These schemes consistently demonstrate her preference for psychological warfare and infiltration over direct confrontation.

Key relationships shape her development significantly. Her bond with Buguese remains central, as she continues to have faith in him even as she loses faith in Mantid. After being injured and becoming a fugitive, she is found and cared for by Grasshop and Princess Sparkle, who calls her Auntie Bee. This encounter challenges her worldview, as she admits to Grasshop that while she is no longer part of Mantid's army, she still harbors hatred toward humans due to her training. Nevertheless, she provides Grasshop with crucial information about the Labyrinth being the only way to reach the Invectid Nation before departing.

Beerain's development traces a path from a cunning antagonist to a reluctant ally. Her wings possess the ability to regenerate when given time to recuperate. In the final confrontation, she joins Buguese and Stags to protect the Spider Riders from Mantid, and they deliver the Oracle Keys in their possession to the protagonist Hunter. After Mantid is defeated, Beerain watches as the sun shines upon the Invectid Nation for the first time in over a hundred years, achieving the goal she had fought for throughout the series.