Shinji is somewhat comical, adding a bit of levity to situations that are otherwise serious. He has a flippant attitude under normal circumstances and seems to like annoying his associate, Hiyori Sarugaki, which usually earns him a slap from one of her sandals. She, in turn, shows him absolutely no respect and frequently insults and attacks him; although he is no better, frequently making faces or quick insults at her expense. He also appears to have a history of referring to cute girls he meets as his 'first love', complete with a deadpan look on his face despite the obvious lie (he apparently never did this with Hiyori, further annoying her). He also apparently enjoys listening to jazz music, as he had a phonograph that played his vinyl records in his days as a Captain.
Underneath this, though, he is quite perceptive in terms of regarding other people, and during his time as a captain, was one of the few people who appeared suspicious of his then-lieutenant Aizen, and could apparently sense or see through some of his illusions. He also quickly notes parts of Urahara's personality. Aizen even admitted to being somewhat intimidated by him. Whenever angered or in combat, Shinji is fairly calm, serious, and level-headed. During a conversation with Ukitake, it was implied that he had been a long-running captain. However, he is not included in Shunsui Kyōraku's list of captains from 100 years before (now 200 years before, because that was said during the Pendulum arc).
Approximately 110 years ago Shinji in his capacity as the 5th Division captain was seen getting ready to attend a captain's promotion ceremony and greeting his lieutenant Sōsuke Aizen upon his arrival. Shinji questioned Aizen's sense of fashion upon seeing that he was only wearing his standard Shinigami robes, prompting the two to start a debate of the necessity of wearing different clothes considering the event. When Aizen commented on the music Shinji was listening to, Shinji explained that it was jazz and it was all the rage in the world of the living at the time. The two made their way to the First Division Headquarters to attend the promotion ceremony.
Upon the doors opening Shinji was ambushed by lieutenant Hiyori Sarugakiof the 12th Division, though she was soon reprimanded by captain Love Aikawa of the 7th Division. When Shinji finally gathered himself he began to taunt Hiyori enraging her even further. As the group proceeded inside Shinji asked Love what the head count was looking like for the ceremony, to which Love responded that almost everyone was already there. Shinji stated that he saw that no one from the 11th Division had shown up. Love explained that was the Eleventh Division's Captain's way, as he did what he wanted. Shinji replied by saying just because he was the 10th 'Kenpachi' no one had the guts to keep him in check. He went on to say that he couldn't understand why they let such a loose cannon like him become captain. Love tried to ease him explaining that there was no use getting upset over it, as the chosen 'Kenpachi' had always held the position of the 11th Division captain. As it was a sort of legacy and it would not change any time soon.
Love went on to say that if Shinji wanted to complain about anything then he should complain about the fact that the previous Kenpachi's ineptitude made it possible. The two captain's conversation was interrupted by the arrival of the 8th Division captain Shunsui Kyōraku being accompanied by his lieutenant Lisa Yadōmaru and 13th Division captain Jūshirō Ukitake. Shunsui accused the two of gossiping behind others backs. Shinji then bore witness to the conversation between Shunsui and Ukitake about who had been captain the longest and what had happened to the previous 12th Division captain Kirio Hikifune. He made note that they were going through captain's quicker than toilet paper. Shinji was later seen waiting in line with the other captains as the newly appointed 12th Division captain Kisuke Urahara arrived to the meeting. He was noticeably unimpressed at the demeanor and first impression Urahara made at his promotion ceremony.
Later the following night he approached Urahara and told him why Hiyori was giving him problems as she had looked up at her previous Captain as a mentor and a mother-figure. Shinji continued by giving him advice on how best to go about being a captain. He then remarked to Urahara that he could tell he wasn't the type who did what other people told him anyway, so it didn't matter what he said. Either way Shinji explained that he felt Urahara was the same kind of man as like him, so he should just meddle in his own business. While leaving Shinji asked, "How long do you plan on watching us from over there". Using his hand Shinji revealed Aizen concealed behind some form of camouflage, Aizen complimented his captain and asked him when did he realize he was there, to which Shinji replied "When you were still in your momma's womb", he then ordered Aizen to follow him. Aizen then commented on how scary a person Shinji was, to which Shinji replies that he was the scary one.
9 years later , Shinji and Aizen were walking around the Seireitei when they came across Urahara, Hiyori and 3rd Seat Mayuri Kurotsuchi. While Shinji once again engaged in a childish fight and argument with Hiyori, Aizen asked Urahara if he had heard the news to which Urahara tells him he hadn't. Shinji proceeded to take up the tale and told Urahara about the series of strange deaths that had taken place in Rukongai. Shinji also informed him that the 9th Division had been sent to handle the investigation.
Later that night Shinji was dispatched with Love Aikawa, Rōjūrō Otoribashi ,Hachigen Ushōda, and Lisa Yadōmaru to investigate the disappearance of Rukongai residents, captain Kensei Muguruma and the members of his division by captain-commander Genryūsai Shigekuni Yamamoto. Upon arrival at the investigation sight Shinji fended off what appeared to be a Hollowfied Kensei from attacking Hiyori, who had been sent there to collect samples by request of the 9th Division.
With the arrival of the rest of the investigation squad, they pondered whether the creature was actually Kensei with Shinji stating that he didn't know if it was Kensei but that if they didn't draw their swords they would be as good as dead. After Kensei let out a loud roar Shinji picked up Hiyori and backed up, he then watched as Love proceeded to engage Kensei in battle. When Hiyori pleaded with them to stop fighting because it was Kensei she began to cough. Shinji told her to be quiet as they had to stop him especially if it was Kensei. When a Hollowfied Mashiro Kuna appeared behind Rose and attacked him, Shinji called out to her so that her attention would be directed at him. Mashiro proceeded towards Shinji and attacked him but he blocked her. He was assisted by the timely arrival of Hachi who used Bakudō #75. "Gochūtekkan" to subdue her. Hachi commented on how fast the others were in getting there. Hachi noticed Love having a hard time fighting a Hollowfied Kensei and used Bakudō #63. "Sajo Sabaku" to restrain him as well. He then asked Shinji what was going on and what had happened to Mashiro and Kensei, but they were interrupted by Kensei breaking free.
As Kensei proceeded to attack the others Hachi used Bakudō #99. "Kin
" to finally bring Kensei down. Shinji commented to Hachi that he was impressed that he used a level 99 binding spell without the incantation. Shinji then asked Hachi if he could fix Kensei and Mashiro with Kidō, but Hachi plainly told him that he couldn't if he didn't know what the problem was. Suddenly Hiyori began to cough uncontrollably, Shinji asked if she was alright and told Hachi to start with her first but he was interrupted by Hiyori who began to choke out that she wanted him to let her go.
Hiyori then Hollowfied and attacked Shinji with a slash across his chest much to everyone's disbelief. It was then that a darkness falls around the rescue squad and they were attacked by an unseen assailant. Shinji was the only one conscious after this and upon the darkness lifting he saw the assailant to be Kaname Tōsen, Muguruma's 5th seat. Shinji asked him how could he betray his captain, it was then that Shinji's attention was caught by the arrival of Aizen and Gin Ichimaru. Aizen explains that he had not betrayed anyone and that he was very faithful and he had just faithfully followed his orders.
Shinji admitted he wasn't as surprised as he let on and Aizen asked if he suspected him and stated that he had expected that much from his captain. Shinji then explained how he knew something was dangerous about Aizen and that he couldn't be trusted. It was for those reasons that Shinji chose him to be his lieutenant and so that he could keep an eye on him. Aizen then told him that he was grateful, as due to his deep seated doubts he didn't suspect a thing. When Shinji stated that he already made it known that he had suspected him, Aizen elaborated stating that he didn't realize that the whole past month, it wasn't Aizen walking behind him. Aizen proceeded to explain the power of his Zanpakutō, Kyōka Suigetsu. Aizen informed Shinji that if he was as close to Aizen as other captains were to their lieutenants he would have been able to see through it by picking up on subtle differences between Aizen and the man acting as Aizen's double.
Shinji asked Aizen what Hollowfication is but Aizen refused to speak on it any further. He then ordered Tōsen to finish them and after Hiyori was attacked, Shinji faced off against Tōsen. Shinji stated that he did not know what was going on but he assumed that nothing had gone exactly as planned just yet. He then remembered that Aizen said agitation speeds up the process, so he resolved to remain calm and level-headed. He began to battle Tōsen beating the traitor back. Shinji continued to battle and soon gained the upper hand sending Tōsen crashing into the ground. But before Shinji could continue the fight the Hollowfication process advanced further and incapacitated him.
Just as Aizen was about to finish off Shinji, captain Kisuke Urahara and Kidō captain Tessai Tsukabishi of the Kidō Corps, appeared and saved him from destruction. Aizen, Gin and Tōsen made their escape and Tessai used Forbidden Kidō to preserve and transport the rescue squad to the 12th Division barracks. In his lab, Urahara tried to reverse the process using his created item, the Hōgyoku. Unfortunately, it ended up as a failure, not having the desired effect Urahara expected.