Shinji is a loyal ally and is always ready to help Kazuma Kiryu whenever possible behind the scenes. He also has a somewhat perverted side, enjoying going to soaplands in his spare time and even trying to persuade Kiryu to get laid.
He wears a black leather longcoat over a purple dress shirt with flower motif, a pair of black slacks and black leather shoes.
Shinji Tanaka was member of a biker gang. He bears a resentment toward yakuza after loan sharks took advantage of his father, tearing his family apart. After separating from his father, he sought to make money by mugging yakuza.
He appears as an opponent in Kiryu's substory A Shining Example, wielding an iron pipe. After he is beaten, Shinji is sold out by a member of his gang to the Jinsei family and depends on Kiryu to save him. After having a talk with Kiryu, Shinji aspires to follow and be just like Kiryu.
Shinji starts as a Dojima Family member working under Kiryu. After the dissolution of the Dojima Family, Shinji joins Akira Nishikiyama's Family becoming a Lieutenant. While many of Kiryu's old friends forgot about him while he was in prison, Shinji did not. After Kiryu returns from prison, Shinji shows where his true loyalties lie by helping out him and Shintaro Kazama.
When Kazama gets shot in Chapter 3, Shinji harbors him from Nishikiyama and the others that are after him.
Later, in Chapter 10, Shinji helps Reina escape from Nishikiyama and this whole chapter involves finding Shinji and trying to save him from hoards of Nishikiyama Family members.
At the end of Chapter 10, Shinji dies from gunshot wounds most likely inflicted by Arase's subordinate Mochizuki.