Saitō Yoshitatsu, the son of Saitō Dōsan, is fueled by whispers that Toki Yorinari—the former ruler of Mino displaced by Dōsan—is his true father. This uncertainty breeds deep resentment, particularly when Dōsan names Oda Nobuna his heir over Yoshitatsu. Possessing a large, sumo wrestler-like build, he towers over others including Dōsan, sports a rugged beard, and is always armored in battle. A later fatal illness, identical to Dōsan's, causes his body to slim and shrivel, revealing a striking resemblance to Dōsan's younger self—confirming their biological bond.

Yoshitatsu's personality undergoes significant change. Initially bitter and paranoid, he reverses Dōsan's pro-trade policies to court Mino's nobility. He distrusts gifted subordinates, especially Takenaka Hanbei, whose strategic brilliance and supernatural shikigami unnerve him; in the light novels, he flees Inabayama Castle upon encountering them. Despite this, he demonstrates tactical cunning, deceiving rivals like a Takeda strategist and rallying troops through intimidation when morale wanes. His resentment fixates on Nobuna as a usurper.

His rebellion forms a critical early arc. Learning of Nobuna's designation, Yoshitatsu murders his younger brothers and launches a coup, triggering the Battle of Nagara-gawa. Though Dōsan escapes with Yoshiharu Sagara's aid, the betrayal leaves Owari vulnerable to Imagawa invasion. Later, during the Mino invasion, he leverages political maneuvering to consolidate power but grows isolated, distrusting allies like Azai Nagamasa whose manipulations he detects.

Following defeat by Nobuna, Yoshitatsu experiences profound transformation. He acknowledges Dōsan's wisdom in choosing Nobuna, apologizes to Hanbei for his mistrust, and prioritizes his men's survival over personal honor. His redemption culminates when, terminally ill, he aids Dōsan in repelling Takeda Shingen's assault on the Oda clan—a pivotal act altering the war's outcome. Seeking peace in his final moments, he journeys to a Buddhist temple accompanied by the repentant sniper Sugitani Zenjūrō. His dying wish is burial alongside Dōsan, symbolizing reconciliation.

Yoshitatsu's combat abilities are inconsistently portrayed. Initially reliant on intimidation, he later exhibits martial prowess, battling Oda generals like Shibata Katsuie and Nobuna in the anime. His tactical acumen shows in manipulating Mino's nobility and deceiving Takeda strategists. Despite his illness, he demonstrates formidable willpower, fighting vigorously against the Takeda and journeying to his chosen death site.