Hartmut is most famously known as the founder of Bern and the original wielder of the Sword of Seals, as well as the Divine Weapon Eckesachs, and one of the Eight Legends that defeated the dragons during The Scouring. After founding Bern, Hartmut constructed the Shrine of Seals so to protect his mighty sword from ever falling into the wrong hands. He constructed it so that the sword could only be drawn in conjunction with the Fire Emblem.
Hartmut has no definite class, as he is only mentioned and never appears, but is speculated to be a King, as that is the only class that is known to wield the Eckesachs as well as his position as the first ruler of Bern. He is, however, mentioned to be a Hero.
During the Scouring, man fought dragon in a savage war that the dragons almost won due to artificially spawned War Dragons. Using the Divine Weapons, man was able to pierce the dragon's scales, how ever the weapons' great power caused the Ending Winter, which rewrote the laws of nature and forced the dragons to take a humanoid form. Eventually, mankind learned that the only dragon that could efficiently produce more War Dragons was the Demon Dragon, Idenn. Using the Divine Weapons, the eight heroes fought their way into the Demon Dragon's lair and found a young girl, who transformed once the heroes attacked, and Hartmut used the mighty Sword of Seals to defeat the dragon. However, Hartmut could not bring himself to kill her, but at the same time could not allow her to reap slaughter on his own species. Hartmut decided to simply seal her away using the Sword of Seals.
This memory was very traumatic to Hartmut and was burned into his mind so profoundly that it served as the memory he passed on through the sword, for all sacred blades can store the memories of their masters. This memory was what Roy saw when he drew the Sword of Seals from its pedestal in Fire Emblem: Binding Blade.
He makes a short appearance in the manga Fire Emblem: Hasha no Tsurugi, where it reveals that he unknowingly married a dragon and had a son together, Al. When he found out that she was a dragon, he sealed mother and child with the Sword of Seals.