Fern is a young human mage who serves as both the apprentice to the elven mage Frieren and a central member of the traveling party in the story. She is a war orphan originally from the southern lands, having lost her parents to conflict. In her childhood, she was on the verge of taking her own life by jumping from a ravine when she was rescued by Heiter, a priest who had been a companion of Frieren on a previous decade-long quest. Heiter adopted Fern and raised her, and he began training her in magic to help her become more self-sufficient. When Fern was nine years old, Frieren came to visit Heiter, and Fern requested that Frieren teach her magic so she could one day become a full-fledged mage in her own right. By the time she turned sixteen, following Heiter's death, Fern embarked on a journey as Frieren's apprentice, a path that would define her role in the story.

In terms of personality, Fern is known for her remarkable maturity, a trait forged by the hardships of her upbringing. This often leads her to act as a responsible caretaker, a role in which she frequently has to manage Frieren herself by waking her up, making sure she is fed and properly dressed, and cleaning up after her. This dynamic has led to fans affectionately referring to Fern as the mother of the party. Despite this serious and dutiful exterior, she is prone to being easily upset and can display a childish streak when she feels wronged, sometimes resorting to pouting or giving others the silent treatment, a situation that often requires an offering of food to resolve. She is generally reserved and does not readily externalize her feelings, yet her irritation is often quite visible to those around her. Fern has a tendency to follow and observe others when she is suspicious or curious about them rather than confronting them directly, as seen when she secretly observed Frieren to see if she was making poor financial choices.

Fern's core motivation is deeply intertwined with her past and her profound sense of gratitude. After being saved by Heiter, she dedicated herself to becoming strong and self-reliant so that he would not have to worry about her future and could pass away in peace without any regrets. This desire to repay kindness and alleviate the burdens of those she cares for extends to Frieren. Even when asked why she continues to seek out magic, a pursuit she does not seem to have an overt passion for, Fern replied that she does it simply because seeing Frieren happy makes her happy as well. Loyalty and discipline are among her most defining traits, as she has trained relentlessly since childhood to become a powerful mage recognized for her abilities.

Her key relationships shape much of her journey. With Heiter, she shared a bond as a father figure, and his teachings continue to influence her long after his death. Her relationship with her master, Frieren, is complex; while Frieren is like an older sister to her, Fern often ends up taking on a near-maternal role due to Frieren's own carelessness, yet their strong bond forms the emotional bedrock of the quest. Fern's dynamic with the warrior Stark is a significant part of her development. Initially, she was cold toward him, viewing him as a hopeless coward. However, as they journeyed together, their relationship gradually improved, marked by bickering and Fern scolding him for his antics, but also by growing concern, mutual understanding, and the emergence of romantic feelings between them. She also shares a notable relationship with the great mage Serie, as Fern is one of the few individuals capable of perceiving that Serie was suppressing her mana, a feat that impressed Serie enough to invite Fern to become her apprentice, an offer Fern declined out of loyalty to Frieren.

Fern's role in the story evolves significantly over time. While she is initially the student who must be guided, her competence and reliability quickly make her an indispensable pillar of the party. She passes the First-Class Mage examination, a testament to her skill and a milestone that allows the party to continue their journey north. Her character development is marked by a gradual opening of her heart. Early on, she is strong and reliable but highly reserved. As the journey progresses and her bonds with her companions, particularly Stark, deepen, she begins to show more vulnerability and honesty with her emotions, a change that is depicted as significant step forward for her character.

Fern possesses a number of notable abilities that set her apart as an exceptional mage. Her primary area of expertise is an extreme proficiency in the offensive magic known as Zoltraak, which she has practiced since a young age and can cast with incredible speed. While she may be inferior to Frieren in terms of raw mana capacity and the number of techniques at her disposal, her true strength lies in the rapid release of her spells. She can cast multiple instances of Zoltraak simultaneously and in rapid succession, allowing her to overwhelm opponents in battles of attrition. She is also an expert in mana concealment, a skill she has maintained since childhood. This ability allows her to approach powerful foes, including demons and accomplished human mages, without being detected until she is right next to them. Frieren herself has admitted to having trouble locating Fern when she is actively concealing her presence. Conversely, she is a master of mana detection, arguably more skilled than any other human in the story. Her ability to sense minute instabilities in the mana of a mage as powerful as Serie was a feat that even much older and more experienced mages could not achieve. Despite her formidable combat prowess, the legendary spell she chose as a reward for passing the First-Class Mage exam was not for combat, but a practical magic to make clothes clean and spotless, a choice that reflects her pragmatic and domestic nature.