Eisen is a dwarf warrior and a former member of the hero's party that defeated the Demon King, having journeyed alongside the elf mage Frieren, the human hero Himmel, and the priest Heiter. As a dwarf, his lifespan, while not as boundless as that of an elf, is far longer than a human's, allowing him to remain alive and relatively healthy long after his human companions have passed away from old age.

In his youth, Eisen was known for being disrespectful and blunt, once nearly causing the party to be executed by a king he failed to properly greet. He is generally taciturn and reserved, often serving as the straight man in the group, but his comments are frequently sharp and insightful. While he can appear careless and rough, this is partly due to his immense physical hardiness, as he is known to survive extreme falls and even having his head gnawed by a dragon without injury. Initially, Eisen held a very pragmatic view of death, believing that the deceased simply return to nothingness and keeping his parents' remains in a common grave with a simple marker. However, after hearing Heiter express that it is kinder to believe the departed are in heaven, Eisen adopted the practice of praying at their graves, a custom he continues to follow even in his old age.

Eisen's primary motivation during his youth was to defeat the Demon King and bring peace to the world. His family was killed by demons, an event that drove him to become a warrior. After the successful quest, he settled into a quiet life. Decades following Himmel's death, Eisen took in a young human boy named Stark, becoming his mentor and teaching him everything he knows about being a warrior. Despite his gruff and sometimes clumsy demeanor, he genuinely cares for his disciple.

In the present-day story, Eisen plays the role of a retired, living legend. When Frieren visits him on her new journey, he is visibly aged and has lost much of the muscle mass from his prime, declining her invitation to rejoin the party because he feels he is too old to swing his axe effectively. He is the one who entrusts Stark to Frieren's care, believing the young warrior will grow strong by traveling with her. His relationship with Frieren is one of deep, quiet understanding between two long-lived companions who share a history of world-saving adventure. His bond with Himmel was one of absolute trust, with the hero placing his very life in Eisen's hands as the party's stalwart vanguard. A key, poignant detail in his relationship with Stark is that they parted on difficult terms; Eisen, catching a glimpse of Stark's latent potential during a fight, was frightened and reflexively struck him, leading to a rift that is eventually mended through understanding.

Eisen is renowned for his superhuman durability and immense physical strength. He is celebrated as the strongest warrior of his era, a reputation known even among demons. His body is so robust that he can withstand powerful poison arrows meant to fell dragons and emerge completely unharmed. He famously wielded a double-headed battle axe that was several times larger than his own body. His defensive capabilities were such that he could block a surprise attack from Rivale, a demon known as the strongest warrior among their kind, a feat that earned the demon's respect and a desire to face him in battle again.