Gensei was an old man wearing a samurai-like outfit. He was balding but had long grey hair on the back of his head, a small mustache and a long beard. He had a noticeable scar over his right eye, as well as long eyebrows.
In his youth, his hair was dark in color.
Gensei loved to fight and kill. In fact, he could only feel alive while he was doing so. He was a calm and composed man and praised his former pupil Bulat for improving; even before dying by Bulat's hand, Gensei stated that there was no greater satisfaction for him than to die in a duel.
In the past, Gensei was a martial arts master in the Imperial Army who trained Bulat, as well as the former user of Incursio. He was regarded as the strongest in his time, decades before Budo was around. After his retirement, Gensei couldn't find a place for himself before realizing that he could only feel joy in life while killing others while on his journey, killing bandits attempting to rob him. After meeting three like-minded individuals, he joined their group called White Brows Association.
Sometime later, their group became very active, taking every request they could, including children, which drew Night Raid's attention. Around that time, Gensei met Tatsumi, Akame and Bulat and invited them into his secluded temple. After finding out that Gensei was one of the White Brows' assassin, Bulat went to fight him and managed to kill him after a long battle.
Gensei was a skilled martial artist and a swordsman well-versed in the art of Iaidō, which allowed him to slash his victims so quickly that they failed to notice it before dying. He displayed his prowess by cutting through Incursio's armor, which is considered to be indestructible by regular weaponry. Gensei also wielded the said armor in the past and was presumably skilled with it, enough to be regarded as the strongest in the Imperial Army in his prime. He also had great instincts in addition to knowing about Incursio's tricks which let him sense and fight Bulat even when the latter was using the Teigu's invisibility.