Eisen is a dwarf and was the warrior of the legendary party of heroes that defeated the Demon King. His name is derived from the German word for iron, a fitting description for his incredibly sturdy and resilient body. As a dwarf, Eisen possesses a long lifespan that, while not matching that of an elf, far exceeds that of a human. Because of this, he is still alive many years after the deaths of his human companions, the hero Himmel and the priest Heiter, showing little change in his appearance.

Eisen carries a tragic past, as his home village was attacked by demons and his family was killed. This trauma initially made him cold, strict, and pessimistic. These traits manifested as a critical stance towards Himmel's desire for a fun and memorable journey, with Eisen often reminding the party of the grim reality of the world and the cruelty of war. He also had a pragmatic view on death, initially not believing in an afterlife and seeing no point in religious practices, a perspective that changed after a conversation with Heiter. He adopted the practice of praying at the graves of his family, a custom he continued even in his old age.

Despite his gruff exterior, Eisen is a man of deep sentiment. He is reserved and speaks little, often playing the straight man to the flamboyance of Himmel and the eccentricity of Frieren. However, moments of joy and playfulness emerged during the journey, such as when he was seen running through a field of flowers and making flower crowns like a child. He maintained a close friendship with Heiter after their journey, exchanging letters frequently until Heiter's death. He also harbors a wish for Frieren and Himmel to be reunited in the place where souls are said to rest, an understanding born from witnessing the painful gap in time and emotion between the elf mage and the human hero.

Eisen acted as the vanguard and frontline fighter for the Hero Party, wielding a massive axe with ease. His most defining characteristic is his superhuman durability. He is completely unfazed by a dragon's bite or a poison potent enough to kill a dragon. He can survive being slammed into the ground from any height during freefall without injury, and has even been shown running across the surface of water. When asked the secret of his toughness, Eisen simply attributes it to "strength of will" or "spirit". His feats of strength are just as impressive; even in his old age, he can effortlessly lift massive boulders. He is humble about his abilities, rejecting the title of "strongest warrior" by stating that everyone stronger than him simply died. His philosophy as a warrior is centered on survival and tenacity, summed up in the belief that the warrior who remains standing at the end is the victor.

Because Eisen was the only warrior Frieren traveled with for centuries, he single-handedly warped her perception of the entire class. She came to genuinely believe that all warriors can survive lethal poison, terminal falls, and other impossible physical feats.

Following the death of his companions, Eisen took in a young human boy named Stark, becoming his master and raising him like a son. He trained Stark in all his combat techniques and life lessons on perseverance and bravery. A falling out occurred when, during an argument, Eisen was startled by a glimpse of Stark's immense potential and reflexively struck him with great force. The incident, which Eisen deeply regretted, led to Stark leaving. Years later, Eisen asked Frieren to recruit Stark as her party's warrior, hoping to give his disciple the chance to have an adventure worth remembering. He described his own time with the Hero Party to Stark as a very silly and very fun journey, revealing how much he ultimately treasured those years.