Matsudaira Motoyasu rules Mikawa Province, initially subordinate to Imagawa Yoshimoto before securing independence after the Battle of Okehazama. She subsequently joins Oda Nobuna's alliance, ascending through tactical brilliance and opportunistic maneuvering to become its second-most powerful member. Her personality blends shyness with sharp intelligence, underpinned by a deep-seated fear of Nobuna rooted in childhood trauma. This fear, born from Nobuna subjecting her to perilous "games" like setting her on fire—requiring repeated rescues by Inuchiyo—manifests as pronounced obedience in their interactions.

Despite her reserved nature, Motoyasu harbors a capacity for intense, sustained rage when pushed beyond tolerance, erupting into uncontrollable fury that even her trusted ninja Hattori Hanzo cannot quell. Childhood experiences of being "passed around like a football" and treated as expendable fostered chronic insecurity and frustration over her perceived lack of autonomy and respect. These emotions fuel her ambition to prove herself worthy of adopting the Tokugawa name, symbolizing her quest for legitimacy and independence.

Motoyasu employs calculated deception to advance her objectives, exemplified by tricking Yoshimoto into signing an unfavorable contract with Nobuna. She demonstrates strategic pragmatism, even considering the exploitation of Nobuna's romantic feelings for Yoshiharu Sagara as a tactical option. Her military acumen surfaces in key campaigns: besieging Oda fortresses Marune and Washidzu under Imagawa's command, deploying ninjas for sabotage, and later opposing the Takeda army's advance almost single-handedly while Oda forces were engaged elsewhere.

Her relationship with Yoshiharu evolves from an initial attempted execution to professional respect and admiration. He becomes one of the few capable of reasoning with her during enraged states, alongside Takeda Shingen. After Yoshiharu warns of an impending Azai-Asakura attack, Motoyasu assigns Hanzo to protect him. She later orchestrates his rescue by infiltrating enemy territory with disguised troops following his apparent death, showcasing her resourcefulness.

Motoyasu's tactical overconfidence leads to critical errors, notably during the Tiger of Kai arc. Insulted by Takeda Shingen's disregard, she impulsively pursues the Takeda army against advice and falls into an ambush, requiring Yoshiharu's intervention to survive the rout. Following this defeat, Yoshiharu's encouragement helps her overcome feelings of inferiority; she rallies remaining forces to confront Shingen again, successfully coordinating with Oda troops to surround the Takeda army.

Hanzo serves as her loyal retainer and chief ninja, prioritizing her safety above all. Their relationship reflects mutual trust, with Hanzo occasionally acting beyond orders to protect her interests, such as withholding battlefield intelligence from Yoshimoto. Motoyasu implicitly relies on his efficiency and subtle humor during operations.