Shinjiro is introduced early on in the story of Persona 3 Portable. He is an old member of SEES who left and refused to join back until much later in the game. He is only useable in Tartarus for about a month, during which time the female protagonist is able to begin a social link with him.
If you want to save Shinjiro from his grim fate, you'll need to max out his social link before the deadline. It should also be noted that only the female protagonist can experience his social link, with the male protagonist being unable to change Shinjiro's fate. Shinjiro's social link begins on September 7th, and you will need to max it out by October 4th in order to save him. Failing to finish the social link by that deadline will also leave you unable to level it up any further.
Shinjiro's Social Link
Only the female protagonist can access Shinjiro's social link, which begins on September 7th. After that, he can be found in the dorm lobby on Mondays, Wednesdays, Fridays, and Saturdays. There are no requirements to begin Shinjiro's social link, as the time limit will make it difficult enough to max. Prioritize Shinjiro each time you see him make sure you reach rank nine as quickly as possible. His social link must be completely finished in order to save him.
In rank 9's hangout, Shinjiro mentions that he's missing his pocket watch. To progress to rank 10, you must find the pocket watch, then wait for the next day he is available to give it to him and initiate the level 10 social rank scene. To find the pocket watch, head to Paulownia mall and go to the police station. Inside talk to officer Kurosawa, and he will tell you he recently found the pocket watch and give it to you. Returning the pocket watch to Shinjiro will max out his social link and also be the most significant action in averting his death later in the game.
Effects Of Saving Shinjiro
On the night when Shinjiro normally dies, he still is shot by Takaya, protecting Ken. Instead of killing him, the bullet hits the pocket watch he now has and stops the bullet. While he is now alive, he will be in a coma for the rest of the game. As Shinjiro has been a playable member of SEES for a while, this route still has him unusable during the final sections of the game. While he is saved from death, he is still effectively written out of the game and won't be seen for the rest of the game. Maxing out his social link will also give you access to special hangouts with him on October 2nd and 3rd.