On the outside, Shin is shown to be very honorable and kind individual, but on the inside, he is very twisted and power hungry that he will not hesitate to kill anyone who gets in his way to power. He is shown to be very proud of himself in what he will achieve and do in order to achieve his goals.
Also shown in some flashbacks and when he talk to Akito, he had no remorse nor guilt on what happened to his family, nor even love for his younger brother. However, he seems to be haunted by the deeds he have done as he kept seeing ghosts of his mother, Alice, Maria, and Michele Manfredi and, for a moment, wanted to take his own life to be with them. It seems, to some extent, he regretted his actions.
However, at other times, Shin seems to desire chaos and the destruction of the world as he knew. He is also very smart in his own right, shown when he deduced that Julius Kingsley is Lelouch vi Britannia.
Back when Shin was younger, he was the son of a Japanese family that submitted to the Holy Britannian Empire after their conquest and annexation of Japan. When he was fourteen, Shin had his entire family killed and ordered Akito to die as well, but he didn't and the two ended up separated for the next ten years. During those ten years, Shin was adopted into a Britannian noble family where he had a Britannian noble mother and sister, both of whom he actually cared about. At some point in time, Shin joined the ranks of the Britannian Military and rose to become a general and was eventually deployed to Euro Britannia to assit in Britannia's war against the Europia United.
After the Battle of Narve in which Akito, known as the Ghost of Hannibal, had wipped out all Euro Britannian forces in the area, the Knights of St. Michael are called in and Shin meets with their Grand Master, Michele Manfredi, where he was proud to take on the task of warring against the E.U. and said that the "ghosts" that accompanied the order's headquarters as mentioned by Shin was the ghosts of the revolution that occured two hunred years prior. Shin however, wanted the position of Grand Master and eventually had Michele kill himself as ordered by his Geass and was eventually promoted to the rank of Knight by the Knights of Euro Britannia and became the new Grand Master of the Order of Michael.
At the Euro Britannian capitol city of St. Petersburg in Russia, Shin in knighted by the leaders of Euro Britannian and is officially declared the new Grand Master of the Knights of St. Michael and vows to defeat the Europia United and conquere the EU for Britannia as a means of silencing Andrea Farnese after he shows skepticism of Michele's suicide since he knew him and he would've never killed himself. Later on, Shin is seen overseeing the Battle of Slonim where he watches as the strike force lead by Ashley Ashra engages the W-0 troops of the EU military. Shin eventually comes in and eventually finds out that his younger brother, Akito, was part of the battle and was surprised that he was both alive and a European soldier. Shin then gave Akito the choice to come and join him but, Ryo Sayama came in and shot at him, but missed. Shin was made that his "reunion" with his brother was interrupted but, he eventually withdrew after the EU forces began to advance and decided to retreat.
Shin has proven to be very observant, as he deduced that Zero is still alive and is in fact Julius Kingsley himself after a single conversation with him. Specifically, he noticed the similarity between Zero's methods and the "Ark Fleet" plan Kingsley has put in motion against the E.U. Furthermore, he even figured out Kingsley is in reality the eleventh prince of Britannia after Julius muttered the name Nunnally. Shin also remarks that he and Suzaku are similar. He particularly mentions that Suzaku has darkness inside him, which seems to refer to how the latter killed his father during the attack on Japan that led to its annexation as Area 11, compared to Shin having killed his own parents as well.
This caused Suzaku to try and assassinate Shin, but Jean Rowe and the Knights of St. Michael arrived to stop him. Although the White Knight overwhelmed them, Shin managed to make Suzaku surrender with Lelouch at gunpoint.
Later, Shin and the Knight of St. Michael gained the location of Castle Weisswolf and arrived to lay siege to it. Shin, in the Vercingetorix, overwhelmed the outer defences and past the minefield, but was stopped by the fortified wall created by W-0. However, the other knights arrived and Klaus Warwick contacted them for a formal surrender with the condition of sparing the lives of the inhabitants. However, Shin later reveals his true agenda to Leila, that he wants to cause open war between the 3 superpowers by nuking Pendragon with W-0's Apollos' Chariot missiles in order to kill the emperor.
He then attempted to use his Geass on Lelia, but her own Geass stopped him, causing him severe pain. This caused an angered Shin to try and kill Lelia and Warwick, but the arrival of the W-0 pilots stopped them. There, Shin tried to make Akito kill Lelia with his Geass, but her feelings for Akito won through where he took Lelia and escaped back to Castle Weisswolf.
During the Assault on Castle Weisswolf, in a flashback to Shin's past, he killed his father and mother (the latter who apparently had conceived Akito from another man, who was present at his (Akito's birth). A skull appears to Shin and tells him that if he wants the power of the King, he would become lonely; regardless, Shin accepts his fate and is granted his Geass power. Akito and Shin continue to fight it out fiercely, both with the robots and in hand-to-hand combat after they've done enough damage to each other.
When it's almost over, Leila appears and uses her Geass, which causes Ryo, Yukiya, Ayano, Ashley and both the dead and living people pleading with/demanding Shin to not kill Akito; Shin remains defiant. Just as he is about to unleash the final blow, Jean shows up and is fatally stabbed by Shin's sword; at the same time, she manages to fatally shoot him. She admits her love for him in her dying breath. Akito goes to his older brother, who initially still rejects him. However, Shin witnesses a flashback in which his young self at first rejected to take care of Akito, but after hearing him cry, he changes his mind and comforts Akito, who then tells him "I love you, big brother"; back to the present, this flashback gives Shin a change of heart and he tells Akito that he must live. Shin then dies and is reunited with Jean in death, where he apologizes for killing her.
Shin's Geass was first revealed when he forced Michele Manfredi to commit suicide by telling him to "Take a trip"; in a flashback, he was shown to have killed a multitude of people with it and used the simple order "Die". Shin's Geass is the ability the make those he loves kill themselves or others, stemming from his beliefs that life is painful and a curse. This is described by the spirit of his mother as his wish to free his loved ones from the "curse". This limitation does not appear to be known or understood by Shin, as he tries to use his Geass on Leila, but to no effect. When used, it manifests in his left eye. Eventually, it spreads to his right eye as well.
His brother, Akito, is the only person to have mysteriously lived after being subjugated to the command of dying, due to him being too young to understand the concept of death at the time. Instead of dying, similar to Lelouch planting the command to "Live" in Suzaku, the effect of Shin's Geass remained in his subconscious, and influenced him during battle. This command was also able to influence the other members of the W-0 Unit through a brain raid over the neuro devices they were using. Shin was also capable of sensing when the command activated while he was near Akito. Akito, however, has been able to overcome the Geass, refusing to kill Leila Malcal when Shin tried to make him do so, similar to how Euphemia li Britannia overcame her own Geass to avoid killing Suzaku.
Shin's Geass is also unique in that he did not enter into a contract with someone with a Code like C.C. or V.V. Rather, after he had killed his father, a vision of a skull with symbol of Geass manifested before him and offered him the power of the king.