Soushin

SouShin is the slash ship between Sou Hiyori and Shin Tsukimi from the Your Turn To Die fandom.

Before the Death Game
When Shin was in high school, Sou disguised himself as a student and befriended Shin in order to steal information for his AI. They apparently spent a lot of time together, and in contrast Sou seems to have only briefly meddled in the other participants' lives.

Shin considered Sou a friend despite being afraid of him and knowing that Sou enjoyed making him uncomfortable.

Sou was killed by Alice Yabusame at some point before the Death Game, around two and a half years before.

In Shin's first trial he was told that he had a 0.0% chance of survival based on countless AI simulations of the game. This knowledge caused him to break down and assume the identity of Sou Hiyori in an attempt to escape his inevitable death. He started to imitate Sou's manipulative behavior and personality, although to a lesser degree.

Chapter 3, Part 1
Sou is revived as an AI doll and takes part in the Death Game as Floor Master of the 4th and 5th floors. At this point, the player has decided whether Shin or Kanna survived Chapter 2. Sou has different dialogue depending on which one of them is alive.

Both routes include the discovery of the Shin AI, which Sou keeps on a computer and is very possessive of. He also has a journal documenting his interactions with the AI.

When asked about Sou by Sara, the Shin AI states that Sou is his "scary friend" and tells the story of how they met. He mentions how being around Sou makes him feel uneasy and anxious, and that he can tell Sou takes pleasure in having this effect on him, but doesn't seem to think of it as a deterrent to their friendship. He is also aware that Sou infiltrated his school and was not actually a student there.

Shin lives route
Shin is visibly upset by Sou's revival and views him as an enemy comparable to Sara, who he despises for her role in Kanna's death. He postpones his plan to murder Sara and cooperates with the rest of the group to fight against Sou. When everyone is investigating the 4th floor, Shin is found staring at a portrait of Sou and Kurumada comments that he's obsessed with him.

Kanna lives route
Sou admits that he is disappointed with this outcome as he wanted to kill Shin with his own hands since he was going to die anyway. This disturbs the other characters, and Sou states that they don't understand his "affection" towards Shin.

Fanon
Sou is frequently misinterpreted by the fandom as a yandere character, but it's more accurate to say that he has Antisocial Personality Disorder. Because of this, many fans interpret his obsession with Shin as a twisted form of romantic love, although Sou never actually shows any kind of romantic affection for Shin. His "affection" stems from his interest in Shin as more of a plaything than an actual romantic partner.

In fanon, Soushin is seen as an abusive relationship due to the implications of their past history. Portrayals tend to depict Sou being possessive of Shin, cruelly seeking pleasure out of making him feel uneasy. Shin in the relationship, especially in pre-game portrayals, is seen as someone who latched onto Sou, despite the mistreatment and discomfort he felt when he was with him. The chemistry between the two characters is one of the highlights of the ship, as well as the vagueness of their relationship so far in canon. It allows for many scenarios to be explored through fanon content.

Trivia

 * During the events of the game, Shin is always seen wearing the same scarf as Sou. It is unknown how or why he has it.

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