A senior who also attends Gekkoukan High School like the rest of SEES. Although tough, cool, and acts like a punk, he is actually kind on the inside. He is also Akihiko's childhood friend and one of the original Persona-users and members of SEES (with Akihiko and Mitsuru). He wields giant axes in battle. Shinjiro's Persona is Castor of the Hierophant Arcana.
What is only known about Shinjiro's past is that he grew up in an orphanage and became best friends with Akihiko Sanada, and his sister, Miki. Tragedy struck later on when a terrible fire erupts on the orphanage they lived in, leaving Akihiko's sister Miki to die in the fire. After the incident, they made a promise to be stronger so that they can protect others. This leads to the latter joining SEES, and they meet Mitsuru Kirijo, who was the only existing member at the time. One tragic incident led to Shinjiro's personality change. During the Dark Hour, the three SEES members were chasing a Shadow in a residential area around Port Island. Shinjiro momentarily lost control of his persona and accidentally killed an innocent bystander. That person turned out to be Ken Amada's mother. Shortly after the accident, Shinjiro left SEES, unable to shake the guilt of what he had done, and lived in the streets from then on. Shinjiro found a way to control his Persona, taking special pills that suppress a Persona's power, should the user lose control of it. We later learn that these pills have poisonous side effects that lead to an earlier death than normal people. Akihiko has also been trying to persuade him to re-join SEES, but Shinji always refused the offer. Only when Ken Amada joins SEES he then decides to re-join.
As soon as Shinjiro joins SEES again, Ken Amada plans his revenge for what he did to his mother two years beforehand. During a full moon, while the rest of SEES fought off two shadows, Ken confronts Shinjiro in the exact place where Ken's mother was killed. Shinjiro tells him to go for it, but warns him that if Ken is successful with his revenge, he will end up like Shinjiro. Just before Ken could act, Takaya of Strega shows up and encourages Ken to kill Shinjiro, but tells him he will die anyway because of the pills' side effects. But Takaya intends to kill both of them on the spot, but Shinjiro takes the bullet for himself, thus saving Amada, with Takaya fleeing from the scene. The rest of the SEES members show up too late, finding Ken stunned and shocked from everything that is just unfolding, and Shinjiro bleeding from his gunshot wounds. He tells Ken not to waste his life because he's still young, and got his whole life ahead of him. Shinjiro also tells Akihiko to take care of himself, before dying in front of his friends with the final line, "This is how it should be." After a few days of grieving and deep thinking, Shinjiro's resolve gives Akihiko and Ken the will to keep fighting on until the end.
During the final battle, Shinjiro's voice can be heard by the main character after the rest of the SEES members lend their powers to him, which helped defeat Nyx.
While playing Persona 3 FES, a special cut scene is added featuring Shinjiro. It is accessed from the console in the command room on the dorm's 4th floor. It shows that Shinjiro has a love for cooking, as he is reading a cooking magazine while watching a cooking show in the lounge with Koromaru. He pretends to sleep when Fuuka and Aigis enter the lounge. He is found out after Koro barks which Aigis translates as "he is not sleeping". Shinji lets out a sigh, which causes Koro to whimper as he didn't mean to cause Shinjiro any distress. He tells him not to bother, and promises to make something special for him soon.
In Persona 3 Portable Shinjiro is one of the characters that the Female Protagonist can romance and potentially end up saving him from death by finding his pocket watch, which will block Takaya's bullet and render him in a coma instead.
Shinjiro is voiced by Kazuya Nakai in the Japanese version of Persona 3, while voiced by Grant George in the English version.