Rem Galleu is one of the primary female characters and a Pantherian, a race known for its feline characteristics. She is a young adventurer with long, flowing black hair that reaches her waist, matching her black, fur-covered ears and tail, which are considered rare even among her people. Her sharp, almond-shaped green eyes and composed expression often give her a doll-like appearance, belying a determined will. Physically, she is slender and athletic, with a toned frame that reflects her rigorous training.

Beneath her calm and polite exterior, Rem is a deeply self-conscious individual. She harbors a significant insecurity about her small chest, which is unusual for a Pantherian and which she views as a disadvantage in romantic contexts. While she often appears serious and focused, she is easily flustered by public displays of affection and becomes notably jealous or irritated when others, particularly her companion Shera, engage in flirtatious behavior. She tends to be cordial with acquaintances, but those close to her witness a snarky, quarrelsome, and pouty side. Her emotional vulnerability often surfaces in awkward or intimate situations, leading to visible embarrassment.

Rem carries a heavy and dangerous secret: the soul of the ancient Demon Lord Krebskulm is sealed within her body, a curse passed down through her family line from her mother. Her death would result in the Demon Lord's resurrection, a fate she is determined to prevent. Driven by this burden, she left her family and refused the protection of the Mage's Association, choosing instead to become an adventurer and summoner. Her ultimate goal is to find a way to destroy the sealed soul and free herself from this legacy. To achieve this, she devised a plan to summon a powerful Demon Lord from another world to aid her.

She meets the elf Shera L. Greenwood at the sacred Starfall Tower, where they inadvertently perform a simultaneous summoning ritual. This ritual calls forth Diablo, an overwhelmingly powerful being from another world. Their attempt to enslave him backfires due to his magical equipment, resulting in both Rem and Shera becoming magically bound to him instead. Despite this initial complication, Rem chooses to stay with Diablo, seeing him as the key to defeating the Demon Lord inside her. Her relationship with Diablo evolves from a practical alliance into genuine affection and trust, eventually culminating in marriage.

Her relationship with Shera begins with constant bickering over who truly summoned Diablo and rivalry for his attention. However, this dynamic gradually matures into a close friendship and a supportive, if still competitive, partnership. A pivotal moment in Rem's character development occurs when the soul of Krebskulm is partially extracted from her body and manifests as a childlike girl named Klem. Despite having dedicated her life to destroying this entity, Rem instinctively moves to protect the vulnerable child, revealing her deep-seated compassion and creating a new, complex bond as she takes on a protective, almost maternal role.

As a Pantherian, Rem possesses natural superhuman speed and strong offensive capabilities. She is a talented Summoner specializing in Earth attribute magic, and at the start of her journey, she is approximately level 40, having successfully contracted with seven different summoned beasts. Her summons include the binding Shadow Snake, the powerful assault bull Aslau, the Stone Man, Iron Gorilla, Rock Wan, the wind-attribute Dragonfly, and the fanged Sabre Tiger. In addition to her summoning magic, she later begins training as a Warrior to expand her combat skills. She is also a skilled martial artist, coming from the Gad family lineage of Fist Saints. To enhance her abilities, Diablo provides her with powerful equipment, including the Layer of Secret Stone armor for defense, the Wild Animal's Ring to double her summons' attack and defense, and the Magic Hands of the Beast gauntlets to increase her speed.