Sakon's original name was Shima Kiyooki, and he wandered around aimlessly without any real aim in life. Before the events of the game, he meets Keiji who, as an ex-vagabond, was very similar to himself. After a gamble match and a heated battle, Keiji tells Kiyooki that it's wrong for him to be wasting his talents when the country is stuck in such a turbulent era, and Kiyooki leaves with Keiji's words stuck in his head. It's not until his chance meeting with Mitsunari that he decides to follow Keiji's words and Mitsunari's lifestyle of living for his lord, and changes his name to Shima Sakon (which means 'close to the left', the left being Mitsunari - Hideyoshi's left arm).
Sakon has a rivalry with Shibata Katsuie , who Sakon believes would have been himself if Mitsunari hadn't entered his life. As their roles could have easily been reversed, Sakon can't leave Katsuie to his fate, and tries to convince him that there is something worth living for. In Sakon's drama ending, he reveals that he knew Katsuie before he became the empty man he is now, and he admired everything about him, and so he can't watch him give up on his future without even trying. While Katsuie had originally been sent to assassinate Hideysohi, Sakon instead takes him to defeat Yoshiteru , before admitting at the end of it all that he was betting on his future.
In Sakon's anime ending, Mitsunari is bitter because Hideyoshi and Hanbee left him out of the battle with Ieyasu's forces. They lay siege on Ii Naotora's land, but during Sakon's battle with Naotora, Tadakatsu enters to give her extra support. Sakon doesn't understand why he would be there if Ieyasu is locked in battle with the Toyotomi, until Yoshitsugu presents the likelihood that Ieyasu won the battle and Hideyoshi and Hanbee are in fact dead. Later, when a messenger of the Toyotomi confirm that Hideyoshi and Hanbee were killed in battle, Mitsunari refuses to believe it and runs off to Osaka in search for them, with Sakon in pursuit. When he gets there, Masamune and Yukimura have stormed the castle and Hideyoshi is nowhere to be found, so Mitsunari desperately assumes that they must be waiting for him at his castle at Sawayama and once again runs off. When Sakon finally catches up to him and tries to talk some sense into him, Mitsunari comes to the conclusion that Sakon is a spy sent in his form by Ieyasu to kill him, and goes berserk. When Sakon comes to the realization that Mitsunari will only calm down if he allows Mitsunari to kill him, he stops fighting and lets Mitsunari vent all his anger on him. Mitsunari comes back to his senses and remembers him before landing the killing blow, but Sakon drops dead moments after welcoming him back, leaving Mitsunari distraught