Shirou is Illya's brother, and a normal high schooler with no knowledge of magic. It is unknown what happened to his biological parents, as the Great Fire had never occurred, but he was still an orphan and adopted by Kiritsugu about ten years ago, similar to his Fate/stay night origins. Since Kiritsugu and Irisviel weren't properly entered in the family register for various reasons, he kept the surname "Emiya" as a result.
Personality-wise, Shirou is very similar to the Shirou Emiya from Fate/stay night. He is much more carefree than his counterparts, nice to others, caring about Illya, and very good at cooking (much to Sella's dismay). He is also rather dense to his female friends' feelings to him and he has the general trait of accidentally running into perverted situations, which makes Rin compare him to an eroge protagonist in reference to Fate/stay night. Illya, Chloe, Rin, Luvia, Moriyama, and Sakura are openly attracted to him, though he doesn't show any interest, much to Rin and Luvia's disappointment.
Shirou tends to be treated poorly by the people around him, though he admits that he's gotten used to being at the bottom rung of the ladder, even below Sella and Leysritt. He is often the victim of bad jokes and victim of Magical Ruby's experiments. He is often asking about Kiritsugu's whereabouts, because the ratio of females to males in his house is very unbalanced.
Fate/kaleid liner PRISMA☆ILLYA Edit
Illya's foster brother, Shirou was adopted by Kiritsugu 10 years ago.
Unlike his Fate/stay night counterpart, this version of Shirou knows nothing (or may know) about magic.