Saito is a young man with blue eyes and black hair. He wears a blue, loose, long-sleeved jacket-like top with raised collar. He wore a pair of jeans and a pair of blue, white-colored soled rubber shoes.
Saito's personality is commonly cheery yet he blows up easily when anyone is vulnerable, especially Louise. Though he wishes to go back to his home, he acclimatizes to his peculiar new status comparatively quickly.
When Louise forms her familiar contract with Saito, the runes "" are inscribed on Saito's left hand. The runes signify Saito's status as Gandálfr (ガンダールヴ Gandāruvu, "Gundolf" in Geneon anime, lit. "God's Left Hand"), a legendary familiar with the ability to excel in the use of any object crafted as a weapon simply by touching it, ranging from melee weapons to war machines including a World War II era A6M Zero fighter (it does not extend to weapons crafted for ornamental purposes). As the series progresses, it is revealed that a Gandálfr exists to protect a Void Mage as they cast their spells. After being mortally wounded fighting Albion's army at the end of the second anime season, Saito is revived by Tiffania, losing his Gandalfr power until Louise re-summons him and renews his contract. He is then knighted by Henrietta and made a vice-captain of the student knight corps by Guiche. In the fourth anime season, after Saito becomes the lord of the region called De Ornielle, it is discovered that Saito is not just Louise's familiar, but also Tiffania's familiar. Therefore, it shows that his newly gained familiar ability known as Lifdrasir (God's Heart) that allows him to boost Void magic to the user he touches (his life is drained every time he use this ability). This ability was depleted after fighting the Ancient Dragon according to Derflinger. Saito's weapon is a cheap sword Louise bought, which turns out to be the talking sword Derflinger (Derf) (Voiced by: Tetsuo Gotō (Japanese), Kaiji Tang (English). Derflinger later reveals that he was once the partner of Gandálfr from six thousand years ago, and that it was no coincidence that Saito and Derflinger are now partners. Derflinger was destroyed in Saito's first confrontation with the Ancient Dragon, but later came back stating he was dormant inside of Gandálfr. Saito is named after Gennai Hiraga, who was a Japanese scientist from the Edo period, and his name is written as "ability man". At the end of the fourth anime series, Saito and Louise get married.
- Saito is named after Gennai Hiraga, an Edo period Japanese scientist.
- His name is written as "Ability Man".