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Guren first met Shinya when they were 16-year-olds while attending the same high school. Instead of sitting in front of the class, Shinya intentionally chose to sit in the back next to Guren and told him that he wanted to become friends with him. He also made it clear that he hated the Hiiragi family just as much as he did, and hoped that once they met they'd be able to "stir some trouble up together".
After the teacher in class called Guren a mongrel, Shinya stood up for him and called her out on it, telling her it was unprofessional of her. Shinya would tease Guren constantly while Guren would get annoyed by it.
As the novels continued, Guren and Shinya slowly became trusted comrades who could count on one another on battlefield.
In some fanfiction that includes both this ship and MikaYuu, Guren and Shinya tend to play the role of supportive parents to Mika and Yuu as the younger pair's relationship gradually develops into a romantic one. Having been best friends ever since they were in high school, they have grown quite close over the years. The best friends to lovers trope is one of the major reasons why fans are drawn to this ship, and another is the unique ways in which they show how much they care for one another - Shinya with his playful teasing and Guren with his seemingly harsh words. Guren's past relationship with Mahiru Hiiragi is also featured in many GureShin fanfics, often with Shinya helping Guren recover from his heartbreak before they eventually realize that throughout that process, they have fallen in love with each other.
On AO3, GureShin is the most written ship for both Guren and Shinya. It is the second most written ship within the Seraph of the End tag.