While she appears in the game only during a flashback, her death is the main motivation behind Shu and Nagi's actions during the game.
She was presumably born in Sekishu, the homeland of the Fuka Ninja together with her sister Nagi and their childhood friends Shu, Zen and San. She, like Zen and Shu may have served the House of Hojo and Princess Sakuya, but this is not explicitly revealed.
As they grew up together, Shizuku and Shu fell in love and became a couple. This earned Shizuku the silent envy of her sister.
Some time before the events of the game war broke out in Sekishu (probably against the Nagao clan). During an infiltration gone awry both she and her sister were severely injured and cornered by the enemy. Nagi desperately held to the hand of her sister, who was about to fall to her death from the rooftops of a castle, but Shizuku insisted to her sister to let her go and escape. Eventually, Nagi's strength gave out, and Shizuku fell while asking with her last breath for Nagi to tell Shu she would love him forever.
In Shizuku's memory Shu, aided by Nagi, begins his mission to obtain the Tenma Mirrors and become the demon lord Tenma.