Like most other characters in the Akaneia Series of Fire Emblem, Jagen's personality is not very well-developed, with much emphasis having been placed on his functionality as a combat unit on the battlefield. That aside, like most other knights appearing in the series, Jagen has been portrayed to be an individual who is fiercely patriotic to the country whom he has pledged his allegiance to (Altea), participating in many battles under its banner for decades. His devotion to his country is especially evident in the Prologue of Shadow Dragon, where he is one of few units who can be selected to act as the decoy to ensure that Marth and the rest of his retainers succeed in fleeing Altea. Should he be chosen, Jagen's parting words are ones that are gallantly courageous, where he, in spite of the dire state of his situation, ironically muses over the suitability of his age being a deciding factor in him being chosen as the sacrifice.
More dimensions to Jagen's character are introduced with the onset of Shin Monshō no Nazo, where he frequently engages the Avatar in preambling discussions before the onset of each battle. Through these interactions, one can deduce Jagen to be both wise and prudent, as he constantly imparts sound advice to the Avatar to ensure that they form effective tactical decisions.
During the production of the original Fire Emblem: Ankoku Ryū to Hikari no Tsurugi, Jagen was originally intended to be killed during the storyline, to provide a tragic coming of age moment where Marth would be forced to face the fact that he lost the adult he relied on. This sequence was scrapped from the final game and storyline, likely due to technical constants or the inconvenience of removing a unit from the player's use.