Eina Tulle is a nineteen-year-old half-elf who works as a receptionist and advisor for the Guild in the labyrinth city of Orario. She is of mixed heritage, with a human father and an elven mother named Ain, and also has a younger sister named Niina. Eina's motivation to join the Guild stemmed from a need for stable income, largely to support her mother's medical bills, as her mother's health suffers outside of the elven forests. At a young age, she left home to study at the School District, where she excelled academically and became one of the few students capable of reading Hieroglyphs, the sacred language of the Gods.

In personality, Eina is known for being serious, responsible, and highly dedicated to her work. She possesses a strong sense of duty to protect the adventurers under her care, a conviction forged by a traumatic early experience when her first assigned adventurer, a young woman named Maris Hackard, perished in the Dungeon. This event caused her to cut her hair short and solidified her resolve to prevent such losses, even if it meant becoming famously strict. Her rigorous teaching and study sessions have earned the nickname Fairy Break among adventurers, who find her methods intimidating and demanding; it is noted that Bell Cranel is the only known adventurer to have completed her full course. Despite her no-nonsense professional demeanor, Eina is humble, dislikes drawing attention to herself, and becomes easily flustered by romantic advances or conversations about love, in which she has little experience. She has admitted a personal preference for unreliable men who trigger her protective instincts, as she finds overly capable individuals intimidating.

Her primary role in the story is as the official Guild advisor to Bell Cranel, whom she met when he first registered as an adventurer. She took responsibility for him when others predicted he would not last long, determined to ensure his survival. She treats Bell with a sisterly affection at first, providing him with crucial knowledge, equipment, and constant lectures on Dungeon safety. This relationship forms a key part of her development. Over time, her platonic care evolves into deeper romantic feelings, a shift she becomes conscious of after witnessing Bell's emotional vulnerability and determination in the face of overwhelming defeat. Her bond with Bell is tested significantly, most notably during the Xenos incident, where she felt hurt and betrayed when he kept secrets from her, leading to a rare, emotionally open confrontation about her place in his life. Even when the city was placed under a powerful charm that erased memories of Bell, Eina's emotional connection to him proved resilient, as reading her journal about him caused her to cry without understanding why.

Beyond Bell, Eina has other key relationships. She is a childhood friend and coworker to Misha Flott, who often teases her about her feelings for Bell. She shares a professional respect with the high elf Riveria Ljos Alf, who is a friend of her mother. Her relationship with her younger sister, Niina, is initially distant due to their years of separation, but she reconciles with her later on with encouragement from Bell. Eina also maintains a cautious professional relationship with her superior, Guild Master Royman Mardeel, who is known to make rude remarks.

Eina possesses no notable combat abilities. Her strengths lie entirely in her intellect and administrative skills. Her encyclopedic knowledge of Dungeon layouts, monster behavior, and survival tactics is a vital resource for any adventurer she guides. Her most unique and rare talent is her ability to read and interpret Hieroglyphs, the divine script, which allows her to correctly translate the contents of an adventurer's Falna, or status sheet. This intellectual prowess makes her an indispensable asset to the Guild and a crucial, non-combat support for Bell on his journey.